Include blob sizes in blob_store service dump.

+ Correctly synchronize a couple of methods
  (used for debugging) which were missing the locking.
+ Log entries deleted during idle maintenance window
  which happens at most once every 24 hrs.

Fixes: 158351456
Test: atest --test-mapping apex/blobstore
Change-Id: I5d1a1ffc28b009274d981d1ccdac30092d4c3732