As described in
ZOOKEEPER-1928, a leader can become overloaded if it sends too many snapshots concurrently during sync time. Sending too many diffs at the same time can also cause the overloading issue.
In this JIRA, we will:
- add diff sync throttling in addition to snap sync throttling
- extend the protection to followers that serve observers
- improve the counting of concurrent snap syncs/diff syncs to avoid double counting or missing counting