Add some comments to the Forseti implementation after review.

Convert the Forseti client pool from an anonymous class to a static inner, and clarify the initialization order of the fields in ForsetiLockManager.
ConcurrentLinkedQueue is faster than ConcurrentLinkedDeque, according to their internal documentation. Taking it at face value.
Describe my conclusion that the batched lock-grabbing API should be dropped completely in favor of always taking the locks one at a time.
Rename LockStackingCompatibility to LockReentrancyCompatibility, to keep with the nomenclature.
Note on using SimpleBitSet for reusing client ids.
Lots of other general notes all over the place.
Merge pull request #2171 from chrisvest/2.1-forseti-comments

Add some comments to the Forseti implementation after review.