Introduce a dedicated client flag NBD_RT_WAIT_ON_TIMEOUT to reset
timer in nbd_xmit_timer instead of depending on tag_set.timeout == 0.
So that the timeout value could be configured by the user to
whatever they like instead of the default 30s. A smaller timeout
value allow us to detect if a new socket is reconfigured in a
shorter time. Thus the io could be requeued more quickly.
In multiple sockets configuration, the user could also disable
dropping the socket in timeout by setting this flag.
The tag_set.timeout == 0 setting still works like before.
Signed-off-by: Hou Pu <houpu@bytedance.com>