session: improve disconnect handling

session: improve disconnect handling

If the caller is the session owning thread or the main thread with a
worker barrier sync (cli/api) add an event to the pending disconnects
vector in the session node and entirely avoid using the event queue.
Useful for bursts of disconnects (like an app detach).

If disconnects come from a processes, be willing to retry enqueueing the
disconnect to the event queue multiple times.

Change-Id: Ieece1f1091b713f94c41c703b6e805bc8498816a
Signed-off-by: Florin Coras <fcoras@cisco.com>