crimson: decouple mgr client reconnect and connect reset handling

crimson: decouple mgr client reconnect and connect reset handling

As of now, the following invocation sequence triggers deadlock when
closing crimson-osd's connection with mgr:
  ProtocolV2::dispatch_reset() --> crimson::mgr::Client::ms_handle_reset
--> crimson::mgr::Client::reconnect --> crimson::net::SocketConnection::close
--> crimson::net::Protocol::close()

In the above invocation sequence, ProtocalV2::dispatch_reset() enters the gate
"pending_dispatch" the leaving of which would wait for the complete of crimson::\
net::Protocal::close() which further wait for the complete of the gate's close().

This commit decouples this waiting chain.

Signed-off-by: Xuehan Xu <xxhdx1985126@163.com>