Prevent multiple RPC requests from executing at the same time.
This fixes a bug where the Qt "console" could issue a second CLI command before the first had finished, and that leaves bts::cli with inconsistent internal state (and then crashing).  While this specific problem could be fixed by a lock in bts::cli, it seems likely the same problem exists in other wallet/blockchain/network RPC calls, so we're serializing all RPC calls.
This seems to make the GUI more responsive, for reasons I don't understand.