session: add infra for safe pool reallocs

session: add infra for safe pool reallocs

This is not to be used lightly.

The idea is to forces pool reallocs to be done only on main thread with
a barrier to make sure pools are always reallocated without
peekers/readers. If rpcs are delayed and the pool runs out of elements,
workers will block waiting for barrier and force the realloc.

Consumers of this api should be session layer and transports.

Type: feature

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I533272a29534338935a3fcf7027c0e7af2ca948c