vcl: support multi-threads with session migration

vcl: support multi-threads with session migration

Currently, mutlti-threads only support one dispatch thread and multiple
worker threads, eventually only dispatch thread is a vcl worker and can
interact with epoll.
This patch will register all threads as vcl worker, and then each
thread can interact with epoll now. Moreover, session migration also
supported, such as socket created in thread A and used (bind, connect
and etc.) in thread B.

Type: feature

Signed-off-by: hanlin <hanlin_wang@163.com>
Change-Id: Iab0b43a33466968c1423d7d20faf1460c8589d91