BUG/MINOR: http/threads: atomically increment the error snapshot ID

BUG/MINOR: http/threads: atomically increment the error snapshot ID

Let's use an atomic increment for the error snapshot, as we'd rather
not assign the same ID to two errors happening in parallel. It's very
unlikely that it will ever happen though.

This patch must be backported to 1.8 with the other one it relies on
("MINOR: thread: implement HA_ATOMIC_XADD()").