buf_pool->watch_ fields to start with the common prefix buf_pool_watch,
so that they can be searched easily:

buf_pool_watch_set(): Renamed from buf_pool_add_watch().  The "add" was
misleading, because only one watch can be active at a time.

buf_pool_watch_clear(): Renamed from buf_pool_remove_watch().

buf_pool_watch_occurred(): Renamed from buf_pool_watch_happened().