vcl: refactor vls to minimize lock usage

vcl: refactor vls to minimize lock usage

Type: refactor

- per vls worker private pool of sessions
- deep copy of vls worker data structures on fork
- maintain a global, i.e., heap allocated, and lock protected  pool of
  elements that track sessions that are shared between workers (due to
  forking).

Credit for uncovering the issue goes to Intel team contributing code to
VSAP (Ping, Yuwei, Shujun, Guoao).

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: Id7d8bb06ecd7b03e4134f1cae23e740cf4634649