logd: worst uid record watermark part five

(cherry pick from commit 5bb2972d)

A regression that resulted in increased memory consumption for some
logging patterns because we rarely did merge or leading checks, and
age-out checking. On the last prune cycle, we reset for a full scan.

Add some comments describing the pruning processes.

Bug: 23327476
Bug: 23681639
Bug: 23685592
Change-Id: I22b0f339c9269b006831fda9cefe295a263ebb92