Don't go through double in AmountFromValue and ValueFromAmount

My prime gripe with JSON spirit was that monetary values still had to be
converted from and to floating point which can cause deviations (see #3759
and https://bitcoin.stackexchange.com/questions/22716/bitcoind-sendfrom-round-amount-error).

As UniValue stores internal values as strings, this is no longer
necessary. This avoids risky double-to-integer and integer-to-double
conversions completely, and results in more elegant code to boot.

src/rpcserver.cpp

@@ -11,6 +11,7 @@
 #include "sync.h"
 #include "ui_interface.h"
 #include "util.h"
+#include "utilmoneystr.h"
 #include "utilstrencodings.h"
 #ifdef ENABLE_WALLET
 #include "wallet/wallet.h"
@@ -118,25 +119,21 @@ void RPCTypeCheckObj(const UniValue& o,
     }
 }
 
-static inline int64_t roundint64(double d)
-{
-    return (int64_t)(d > 0 ? d + 0.5 : d - 0.5);
-}
-
 CAmount AmountFromValue(const UniValue& value)
 {
-    double dAmount = value.get_real();
-    if (dAmount <= 0.0 || dAmount > 21000000.0)
+    if (!value.isReal() && !value.isNum())
+        throw JSONRPCError(RPC_TYPE_ERROR, "Amount is not a number");
+    CAmount amount;
+    if (!ParseMoney(value.getValStr(), amount))
         throw JSONRPCError(RPC_TYPE_ERROR, "Invalid amount");
-    CAmount nAmount = roundint64(dAmount * COIN);
-    if (!MoneyRange(nAmount))
-        throw JSONRPCError(RPC_TYPE_ERROR, "Invalid amount");
-    return nAmount;
+    if (!MoneyRange(amount))
+        throw JSONRPCError(RPC_TYPE_ERROR, "Amount out of range");
+    return amount;
 }
 
 UniValue ValueFromAmount(const CAmount& amount)
 {
-    return (double)amount / (double)COIN;
+    return UniValue(UniValue::VREAL, FormatMoney(amount, false));
 }
 
 uint256 ParseHashV(const UniValue& v, string strName)