rpc: add a flush_cache RPC

This allows flushing internal caches (for now, the bad tx cache,
which will allow debugging a stuck monerod after it has failed to
verify a transaction in a block, since it would otherwise not try
again, making subsequent log changes pointless)

src/daemon/command_parser_executor.h

@@ -154,6 +154,8 @@ public:
   bool print_net_stats(const std::vector<std::string>& args);
 
   bool set_bootstrap_daemon(const std::vector<std::string>& args);
+
+  bool flush_cache(const std::vector<std::string>& args);
 };
 
 } // namespace daemonize