kvserver: go through DB for GC

[tbg]

Instead of doing a bespoke thing that hands requests to the replica directly
and bypasses a lot of plumbing, go through the DB. This means GCRequest enters
the Replica like every other request, after passing through the DistSender
stack and being routed through the Node.

This avoids surprises such as discovered in #143122.

Epic: none
Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[sumeerbhola]

Reviewable status: complete! 0 of 0 LGTMs obtained

---

pkg/kv/kvserver/mvcc_gc_queue.go line 603 at r1 (raw file):

	var b kv.Batch
	b.AddRawRequest(&req)

Is this sufficient to initialize all the fields in kv.Batch. For example, we were ad-hoc specifying a RangeID and only initializing part of the ReplicaDescriptor. Presumably all of that will now be computed inside DB.Run, though I can't spot where that happens.

Also, we need to specify the AdmissionHeader too.