chore: Designate semi-private "friend" helper methods as such

This patch implements flaper87's suggestion from several weeks ago
to either define ancillary storage driver methods in the ABC, or
to treat them as "semi-private" and rename them to include an
underscore in the front.

I took the one method that was implemented by both storage drivers, first(),
and formally defined it in the ABC. The remaining mongo helper methods
that support other controller classes within the same driver (i.e.,
"friend" classes) were given the canonical "_" prefix.

Change-Id: I6b4f8ab7f961a621bf85e122ef7e33a3c1084f8f
Merge "chore: Designate semi-private "friend" helper methods as such"