Allow accept callbacks to be short-circuited in primary event-base

Allow accept callbacks to be short-circuited in primary event-base

Summary:
It looks like we were effectively avoiding short-circuiting callbacks submitted for execution in primary event-base (evb == nulptr). The check was there, but it was never effective, since on `addAcceptCallback` we would mask the `nullptr` with our event base pointer.

I see two ways to fix that: either modify the check

    if (info->eventBase == nullptr) { ...} on line 834

to compare to the presently attached event base, or store `eventBase = nullptr` into callbacks_ list (CallbackInfo struct). The second approach requires more changes (implemented here) but allows the caller to still submit callbacks for execution via notification queue event in primary event base by supplying eventBase parameter != nullptr in addAcceptCallback. I therefore chose the second approach.

The existing unit-tests needed modification to avoid using the "broken" nullptr semantics (most cases were assuming it would be using notification queue signaling). I quickly looked at fbcode, and it looks like we only have a few cases of addAcceptCallback() with nullptr, the unit-tests for those are passing.

NOTE: The removeAcceptCallback() semantics is different with regards to eventBase; nullptr here means "scan all callbacks regardless of event-base they belong to".

Reviewed By: djwatson

Differential Revision: D3714697

fbshipit-source-id: 2362bcff86a7e0604914b1cb7f1471fe4d03e78e