VPP-1102: fix dangling references in RPC handling

VPP-1102: fix dangling references in RPC handling

Queue RPC calls and send them from the main dispatch loop. As things stood,
if the vpp main input queue filled, worker threads could enter a
barrier-sync spin-wait in the middle of processing a frame. If thread
0 decided to recreate worker thread data structures, the worker thread(s)
could easily crash.

Legislate the problem out of existence by enqueueing RPC messages only
from the main dispatch loop. At that point, doing a barrier-sync wait
is perfectly OK.

Change-Id: I18da3e44bb1f29a63fe5f30cf11de732ecfd5bf7
Signed-off-by: Dave Barach <dave@barachs.net>