svm session vcl: per app rx message queues

svm session vcl: per app rx message queues

Add option to use per app private segments for app to vpp message
queues, as opposed to exposing internal message queues segment.

When so configured, internal message queues are still polled by the
session queue node but external app message queues are handled by a new
input node (appsl-rx-mqs-input) that runs in interrupt state. Signaling
of the node, when mqs receive new messages, is done through eventfds
epolled by worker epoll input nodes.

Type: feature

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: Iffe8ce5a9944a56a14e6d0f492a850cb9e392d16