Vttablet panic in requests Wait

[GuptaManan100]

Description

This PR fixes the issue pointed out in #14933.

Reading the docs of sync.WaitGroup we see -

// Note that calls with a positive delta that occur when the counter is zero
// must happen before a Wait. Calls with a negative delta, or calls with a
// positive delta that start when the counter is greater than zero, may happen
// at any time.
// Typically this means the calls to Add should execute before the statement
// creating the goroutine or other event to be waited for.
// If a WaitGroup is reused to wait for several independent sets of events,
// new Add calls must happen after all previous Wait calls have returned.

So, apparently, this panic happens, when we have started waiting for a waitGroup and then a bunch of Done() calls happen that take the counter to 0, but before the Wait() has been processed, an Add() call goes through and that leads to this panic.

The only way to properly fix this panic would be to ensure that we don't call Add() after we have started waiting for any request. To this end, we have added a new field to the statemanager that tracks the number of go routines that are waiting for the requests to be empty. We use this field to prevent Add() calls in startRequests.

Related Issue(s)

• Fixes Bug Report: Panic in vttablet in requests Wait #14933

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Documentation was added or is not required

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9df71e0) 47.26% compared to head (fab8966) 47.30%.
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14924      +/-   ##
==========================================
+ Coverage   47.26%   47.30%   +0.04%     
==========================================
  Files        1136     1137       +1     
  Lines      238484   238657     +173     
==========================================
+ Hits       112721   112907     +186     
+ Misses     117158   117138      -20     
- Partials     8605     8612       +7     

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