Fix deadlock between health check and topology watcher

[GuptaManan100]

Description

This PR fixes the deadlock described in #16994.
The problem was that the sending on the channel loadTabletsTrigger in updateHealth was a blocking call. So, in this PR, I've changed the channel from being an unbuffered channel to a buffered channel of a single capacity. I have also updated the sending on the channel loadTabletsTrigger in updateHealth to be unblocking.

This fix has 2 advantages -

1. It fixes the deadlock, since now updateHealth cannot block on sending on the channel. It will either succeed in sending, or just move ahead since the channel would already be full.
2. Not super important, but still, if multiple updateHealth calls happen really quickly, we won't trigger multiple runs of loadTablet since we won't send on an already full channel.

Since this is a fix for a deadlock, it needs to be backported to all release branches since all of them have this issue.

Related Issue(s)

• Fixes Bug Report: Deadlock in health check and topology watcher #16994

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.02%. Comparing base (0fa7e58) to head (944c637).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16995      +/-   ##
==========================================
- Coverage   69.34%   67.02%   -2.32%     
==========================================
  Files        1571     1571              
  Lines      204180   251681   +47501     
==========================================
+ Hits       141582   168688   +27106     
- Misses      62598    82993   +20395     

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[deepthi]

LGTM