vstreamer: support 'useThrottler' so that clients can choose whther they at all want to involve the throttler

[shlomi-noach]

Description

An extension to #13177. This PR includes everything in #13177 and then adds more logic. I intentionally split the two apart so we can discuss the two changes separately.

#13177 says: "if the replica tablet's throttler is checked, then as result the primary tablet's throttler should request more heartbeats".

However, there are some clients that:

1. Engage with the throttler all the time
2. Do not really need the throttler's work
3. Thereby needlessly cause a scenario where the primary throttler leases heartbeats continuously and infinitely. Which works against the idea of "on-demand" heartbeats.

The particular clients are these three members of tabletserver:

• schema tracker
• binlog watcher
• messager

For now, we consider all other clients as "need to use the throttler".

We add a flag useThrottler in VStream(). The callers of VStream() now indicate whether they want to use the throttler or not.

The reason this is done this way is because tabletserver's vstream engine is a single entity shared among multiple clients. It uses a single throttler client.

Related Issue(s)

• Tablet throttler: non-PRIMARY tablets report back to PRIMARY throttler when they've been 'check'ed #13177
• VReplication Workflows and VStream API gets stuck in the copy phase if tablet type is set only as a replica #13175

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

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[shlomi-noach]

I could also go with a different approach: give each client a name, e.g. "schema_tracker", "binlog_watcher" etc. Then, make the throttler itself aware of those clients. So it can choose to let it always pass the check, and also not request new heartbeats.

[shlomi-noach]

I could also go with a different approach

I'm actually leaning towards that approach.

[shlomi-noach]

Closing in favor of #13195