session: avoid debug half-open session pool contention

session: avoid debug half-open session pool contention

Half-opens are only allocated from main with worker barrier but can be
cleaned up, i.e., session_half_open_free, from main without a barrier.
In debug images, the free_bitmap can grow while workers peek the
sessions pool, e.g., session_half_open_migrate_notify, and as a result
crash while validating the session. To avoid, proactively grow bitmap in
debug images.

Type: fix

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I2410793f933fb638651fe8dc08ba78e9bee0bd77