watches in aws. Pending watch notification removal in epoch counters

[EricPyZhou]

1. support exists, get children watches for distributor operations.
2. remove pending watches in the epoch counters and update the node in user storage accordingly.

The full work is comprised of changed made in this repo, and another PR: spcl/faaskeeper-python#13 in the faaskeeper-client repo. But note that, merging this PR alone will not break the system.

Here are the things to be merged into faaskeeper-client repo, please find the details in that PR:

1. partial removal of some watches in a path in add_watch_notifications() in queue.py
2. implement stall mechanism.

The implementation is made with staying compliant with the consistency model that read-after-write always gives the latest result for a single client.