Move "previous coinbase" UI monitoring into ThreadNotifyWallets

When the wallet notification logic was moved into a separate thread,
most wallet notifications were transferred across. This one was missed,
and it is particularly pernicious: all it does is ask the wallet to tell
the UI that a particular transaction had been updated. We don't actually
_have_ any UI connected in zcashd, but there is a side-effect: the
callback blocks on acquiring `cs_wallet`, in the main thread that
already holds `cs_main`. For particularly large wallets, this can cause
the main thread to block on `ThreadNotifyWallets`, which in turn means
that anything waiting on `cs_main` (e.g. RPC calls) is blocked.

We solve this by moving the callback into `ThreadNotifyWallets`. We
don't technically need it for `zcashd`, but we maintain it in case a
downstream fork has reconnected a UI.

src/main.cpp

@@ -3618,14 +3618,6 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
     int64_t nTime3 = GetTimeMicros(); nTimeIndex += nTime3 - nTime2;
     LogPrint("bench", "    - Index writing: %.2fms [%.2fs]\n", 0.001 * (nTime3 - nTime2), nTimeIndex * 0.000001);
 
-    // Watch for changes to the previous coinbase transaction.
-    static uint256 hashPrevBestCoinBase;
-    GetMainSignals().UpdatedTransaction(hashPrevBestCoinBase);
-    hashPrevBestCoinBase = block.vtx[0].GetHash();
-
-    int64_t nTime4 = GetTimeMicros(); nTimeCallbacks += nTime4 - nTime3;
-    LogPrint("bench", "    - Callbacks: %.2fms [%.2fs]\n", 0.001 * (nTime4 - nTime3), nTimeCallbacks * 0.000001);
-
     return true;
 }
 

src/validationinterface.cpp

@@ -250,6 +250,11 @@ void ThreadNotifyWallets(CBlockIndex *pindexLastTip)
             // exploitable as a timing channel.
             GetMainSignals().ChainTip(blockData.pindex, &block, blockData.oldTrees);
 
+            // Notify UI to display prev block's coinbase if it was ours.
+            static uint256 hashPrevBestCoinBase;
+            GetMainSignals().UpdatedTransaction(hashPrevBestCoinBase);
+            hashPrevBestCoinBase = block.vtx[0].GetHash();
+
             // This block is done!
             pindexLastTip = blockData.pindex;
         }