core: Optimize check for pending events

core: Optimize check for pending events

Prior to this commit, a check for whether any events are pending
involved checking four different variables within the context. Optimize
this by using multiple bits within a single unsigned integer to
represent all the possible events that could be pending. This reduces
the check for whether any events are pending to a single load.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>