Changes to support memo sample

Mostly includes convenience functions for determining balance.

src/main/java/cash/z/wallet/sdk/data/MockSynchronizer.kt

@@ -83,7 +83,7 @@ open class MockSynchronizer(
 
     override fun activeTransactions() = activeTransactionsChannel.openSubscription()
     override fun allTransactions() = transactionsChannel.openSubscription()
-    override fun balance() = balanceChannel.openSubscription()
+    override fun balances() = balanceChannel.openSubscription()
     override fun progress() = progressChannel.openSubscription()
 
     /**
@@ -109,6 +109,18 @@ open class MockSynchronizer(
      */
     override fun getAddress(accountId: Int): String = mockAddress.also {  twig("returning mock address $mockAddress") }
 
+    /**
+     * Returns the available balance by adding up all the transactions and subtracting the miner's fee.
+     */
+    override fun getAvailableBalance(accountId: Int): Long {
+        if (transactions.size != 0) {
+            return transactions.fold(0L) { acc, tx ->
+                if (tx.isSend && tx.isMined) acc - tx.value else acc + tx.value
+            } - 10_000L // miner's fee
+        }
+        return 0L
+    }
+
     /**
      * Uses the [forge] to fabricate a transaction and then walk it through the transaction lifecycle in a useful way.
      * This method will validate the zatoshi amount and toAddress a bit to help with UI validation.

src/main/java/cash/z/wallet/sdk/data/SdkSynchronizer.kt

@@ -152,8 +152,8 @@ class SdkSynchronizer(
     /**
      * A stream of balance values, delegated to the [wallet].
      */
-    override fun balance(): ReceiveChannel<Wallet.WalletBalance> {
-        return wallet.balance()
+    override fun balances(): ReceiveChannel<Wallet.WalletBalance> {
+        return wallet.balances()
     }
 
 
@@ -196,10 +196,18 @@ class SdkSynchronizer(
     /**
      * Gets the address for the given account.
      *
-     * @param accountId the optional accountId whose address of interest. By default, the first account is used.
+     * @param accountId the optional accountId whose address of interest. Typically, this value is zero.
      */
     override fun getAddress(accountId: Int): String = wallet.getAddress()
 
+    /**
+     * Gets the available balance for the given account. In most cases, the stream of balances provided by [balances]
+     * should be used instead of this function.
+     *
+     * @param accountId the optional accountId whose available balance is of interest. Typically, this value is zero.
+     */
+    override fun getAvailableBalance(accountId: Int): Long = wallet.availableBalanceSnapshot(accountId)
+
     /**
      * Sends zatoshi.
      *

src/main/java/cash/z/wallet/sdk/data/Synchronizer.kt

@@ -43,7 +43,7 @@ interface Synchronizer {
     /**
      * A stream of balance values.
      */
-    fun balance(): ReceiveChannel<Wallet.WalletBalance>
+    fun balances(): ReceiveChannel<Wallet.WalletBalance>
 
     /**
      * A stream of progress values, typically corresponding to this Synchronizer downloading blocks. Typically, any non-
@@ -89,10 +89,18 @@ interface Synchronizer {
     /**
      * Gets the address for the given account.
      *
-     * @param accountId the optional accountId whose address of interest. By default, the first account is used.
+     * @param accountId the optional accountId whose address is of interest. By default, the first account is used.
      */
     fun getAddress(accountId: Int = 0): String
 
+    /**
+     * Gets the available balance for the given account. In most cases, the stream of balances provided by [balances]
+     * should be used instead of this function.
+     *
+     * @param accountId the optional accountId whose balance is of interest. By default, the first account is used.
+     */
+    fun getAvailableBalance(accountId: Int = 0): Long
+
     /**
      * Sends zatoshi.
      *

src/main/java/cash/z/wallet/sdk/secure/Wallet.kt

@@ -99,10 +99,20 @@ class Wallet(
     /**
      * Stream of balances.
      */
-    fun balance(): ReceiveChannel<WalletBalance> {
+    fun balances(): ReceiveChannel<WalletBalance> {
         return balanceChannel.openSubscription()
     }
 
+    /**
+     * Return a quick snapshot of the available balance. In most cases, the stream of balances
+     * provided by [balances] should be used instead of this funciton.
+     *
+     * @param accountId the account to check for balance info. Defaults to zero.
+     */
+    fun availableBalanceSnapshot(accountId: Int = accountIds[0]): Long {
+        return converter.getVerifiedBalance(dataDbPath, accountId)
+    }
+
     /**
      * Calculates the latest balance info and emits it into the balance channel. Defaults to the first account.
      *