SOLR-6122 POC new cancel handler for Collections api

[QFREEE]

https://issues.apache.org/jira/browse/SOLR-6122

Description

This PR implemented a new overseer level message handler to cancel submitted & in-progress collections api calls. As an example, this PR only opt-in backup collection api for in-progress cancel.

in-progress collections api calls are not complete, as core level cancel is not implemented. Current implemention removes tracking of in-progress collections api from overseers, which can be useful in some situation like backup.

Solution

General flow:
-> Client request cancle with async ID through
http://localhost:8983/solr/admin/collections?action=CANCEL&requestid=1

-> Cancel message handled by OverseerCancelMessageHandler to determine if this asyncId exist and if it's currently in submitted or in-progress state :

• if submitted: remove from workQueue and clear blockedTasks in case submitted task is in blocked tasks
• if in-progress: check if is isInProgressCancelable -> remove in-progress tasks from tracking queues in overseer workQueue & runningMap & runningTasks

Api:
New V2 api interface for cancel handler

Overseer:

• stored asyncId on to ZK's running map to keep track of in-process async task easier
• Adjust OverseerTaskQueue to handle remove / get item from queue given zk path

OverseerCancelMessageHandler:

• a new message handler specifically to handle cancel. Since cancel collections api involovs removing queue event from overseer's queue, added this part of logic as message handler instead of a new collection api

Tests

Test locally through CURL, working on testing

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide

[QFREEE]

@dsmiley hi David, you might be interested in this PR

[dsmiley]

I might be interested but I confess I'm very biased to see the Overseer die. Adding a feature to it is the opposite of that.