Add RPC call abandontransaction

- Make wallet descendant searching more efficient
- Add new rpc call: abandontransaction

Unconfirmed transactions that are not in your mempool either due to eviction or other means may be unlikely to be mined.  abandontransaction gives the wallet a way to no longer consider as spent the coins that are inputs to such a transaction.  All dependent transactions in the wallet will also be marked as abandoned.

- Add RPC test for abandoned and conflicted transactions.
- [Wallet] Call notification signal when a transaction is abandoned

Github-Pull: #7312
Rebased-From: 9e69717254 01e06d1fa3 df0e2226d9 d11fc1695c

src/rpcserver.cpp

@@ -346,6 +346,7 @@ static const CRPCCommand vRPCCommands[] =
     { "wallet",             "getreceivedbyaccount",   &getreceivedbyaccount,   false },
     { "wallet",             "getreceivedbyaddress",   &getreceivedbyaddress,   false },
     { "wallet",             "gettransaction",         &gettransaction,         false },
+    { "wallet",             "abandontransaction",     &abandontransaction,     false },
     { "wallet",             "getunconfirmedbalance",  &getunconfirmedbalance,  false },
     { "wallet",             "getwalletinfo",          &getwalletinfo,          false },
     { "wallet",             "importprivkey",          &importprivkey,          true  },