fixes and improvements

[vipalade]

Version 12.2

• improvements and fixes on reflection
• use std::variant in example_threadpool
• TypeToType -> std::type_identity
• mprpc: add support for ConnectionContext::pauseRead and Connection::Context::resumeRead
• aio: stream fix not call doTrySend or doTryRecv after disconnect
• mprpc: do not reset timer on onSend with error if connection already stopping
• mprpc: add support for ConnectionContext::stop() for stopping the connection