Fix a typo
Document deferred eventcb behaviour
Add watermark introspection
Add an option to trigger bufferevent I/O callbacks
Add an option to trigger bufferevent event callbacks
Clarifications in response to merge req. comments
Merge remote-tracking branch 'mistotebe/bufferevent_trigger'
Make bufferevent_trigger_nolock_() inline

Since most of its callers are using constant EV_READ or EV_WRITE, and
using constant 0 as its argument, this should eliminate most of the
overhead for this function in the fast case.
Minor optimizations on bufferevent_trigger options

By making BEV_TRIG_DEFER_CALLBACKS equal to BEV_OPT_DEFER_CALLBACKS,
and BEV_TRIG_IGNORE_WATERMARKS disjoint from BEV_OPT_*, we can save a
few operations in bufferevent_run_*, which is critical-path.