vlib: epoll on worker threads

vlib: epoll on worker threads

This patch teaches worer threads to sleep and to be waken up by
kernel if there is activity on file desctiptors assigned to that thread.

It also adds counters to epoll file descriptors and new
debug cli 'show unix file'.

Change-Id: Iaf67869f4aa88ff5b0a08982e1c08474013107c4
Signed-off-by: Damjan Marion <damarion@cisco.com>