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.

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  },