id-pool: Re-factor recirculation id allocator into standalone id pool.

Refactor the lock-free portion of the recirculation id allocator
into stand-alone id pool. This is in preparation for re-using
that portion to allocate bucket ids which are part of (draft)
OpenFlow 1.5 groups.

ONF-JIRA: EXT-350
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>