Extended session command support to cover COM_CHANGE_USER, and COM_INIT_DB.
This implementation doesn't guarantee execution order between session commands and queries
if other backend server lags behind in session command execution.

In poll.c : moved processing of EPOLLERR and EPOLLHUP after processing of EPOLLIN and EPOLLOUT.
This ensures that COM_QUIT messages are read and routed forward before signals arrive (from local client/backend).