feat(libmsg): add pymsg C++ library with AsyncQueue, MsgServer, and JSON RPC layer

- Add libmsg module: pymsg protocol implementation (4-byte BE length prefix + UTF-8)
- Add AsyncQueue<T>: generic async batch queue with backpressure (aligned with Python AsyncRpcQueue)
- Add MsgServer: select-based multi-client TCP server with optional thread pool
- Add RpcClient/RpcServer: JSON RPC convention layer with automatic route injection
- Refactor gateway/LogC.hpp to use AsyncQueue for async batch log delivery
- Update gateway/DbC.hpp to add call_msg() using pymsg TCP protocol
- Remove duplicated gateway/inc/Msg.hpp (now provided by libmsg)
- Update gateway/Makefile and _inst.sh build order

Fixes included:
- pmsg_send size check uses kMaxMsgSize (64MiB) instead of 4GiB
- MsgClient._running uses std::atomic<bool> for thread safety
- Deadline-based recv timeout to prevent Slowloris (header+payload shared deadline)

_inst.sh

@@ -29,6 +29,7 @@ inst_module "libth"
 inst_module "libhttp"
 inst_module "libcrypt"
 inst_module "libsqlite3"
+inst_module "libmsg"
 inst_module "gateway"
 inst_module "waf"	# waf 在 gateway 之前编译，部署时由 core 先拉起 waf 再拉起 gateway
 inst_module "static"