BUG/MEDIUM: ssl: force a full GC in case of memory shortage

BUG/MEDIUM: ssl: force a full GC in case of memory shortage

When memory becomes scarce and openssl refuses to allocate a new SSL
session, it is worth freeing the pools and trying again instead of
rejecting all incoming SSL connection. This can happen when some
memory usage limits have been assigned to the haproxy process using
-m or with ulimit -m/-v.

This is mostly an enhancement of previous fix and is worth backporting
to 1.5.