Update for new kernel ABI (stale event handling)

Update for new kernel ABI (stale event handling)

Update to handle new kernel ABI for avoiding stale asynchronous
events.  When a CQ, QP or SRQ is destroyed, the kernel reports the
number of events it has given to userspace, and we wait until we've
handled the same number of events.

This does introduce a library API change: consumers are now required
to call ibv_put_async_event() to release every asynchronous event that
they retrieve via ibv_get_async_event().

Signed-off-by: Roland Dreier <rolandd@cisco.com>