…hreads and fetcher threads (apache#231)

* [LI-HOTFIX] Consolidate the interactions between the RequestHandler threads and fetcher threads

TICKET = LIKAFKA-39536
LI_DESCRIPTION =
In the current code base, the request handler threads need to sync with each fetcher thread
multiple times during the processing of a LeaderAndIsr request, using the
AddPartitions, RemovePartitions, and GetPartitionsCount events.
Each event may be delayed up to request.timeout.ms, i.e. 300s as configured in our clusters.

This PR tries to mitigate the problem by consolidating the multiple events into one
single event ModifyPartitionsAndGetCount for the processing of a LeaderAndIsr request.
Processing of the StopReplica requests has also been migrated to use the
ModifyPartitionsAndGetCount event.

Besides this code change, I also plan to test a reduced value of request timeout
for the fetcher use case.

EXIT_CRITERIA = When this change gets merged in upstream

…the RequestHandler threads and fetcher threads (apache#231)" (apache#243)

This reverts commit a34a489.
During certification, we found that this change is not safe.
For example, suppose follower 0 is currently fetching data for partition tp0 from leader 1,
and we are trying to switch the leadership to leader 2.

In the previous implementation, tp0 goes through the following steps
1. tp0 is removed from the fetcher targeting leader 1,
2. retrieving the fetch offset based on the new leader's epoch. Say leader 2 returns a fetch offset of 100.
3. tp0 is added to a new fetcher thread with the fetch offset of 100.

After the PR, the following race condition may happen
1. tp0 retrieves the fetch offset, e.g. 100, from leader 2
2. tp0 is concurrently sent for removal from the fetcher-with-leader-1
   and add to the fetcher-with-leader-2 with a fetch offset of 100.

Before the removal from fetcher-with-leader-1 is processed, more data may
possibly be retrieved from leader 1 and appended to the log (e.g. making the log end offset to become 102).
Then the fetch offset, i.e. 100, would not match the log end offset, i.e. 102.

…4] (apache#231)

…4] (apache#231)