storage/apply: rename Batch.Commit to Batch.ApplyToStateMachine

[nvanbenschoten]

This came out of a discussion with Dan. The method can't be moved
to StateMachine because then we'll need a type switch to handle the
different kinds of Batch implementations, but it can be renamed to
provide more symmetry with StateMachine.ApplySideEffects.

The commit also tries to clean up some of the phrasing around the
persistent state transition updates and the side-effects of these
state transition updates.

Release note: None

[cockroach-teamcity]

This change is 

[danhhz]

can you elaborate on the type switch? it is similar to the one at the beginning of (*replicaStateMachine).ApplySideEffects or something else?

[nvanbenschoten]

No, this would be a type switch on the type of the Batch passed to the method. A big part of why Batch is an interface is because we're going to introduce an ephemeral batch implementation in the next PR (this was part of #39254 until the very end, but it was pulled before merging because it actually belongs in #38954). This ephemeral batch will return an error from ApplyToStateMachine because it's ... ephemeral.

Making ApplyBatch a method on StateMachine that has specialized behavior for different implementations of Batch instead of letting each implementation of Batch implement an ApplyToStateMachine method seems like the wrong way to go.

[danhhz]

Gotcha, I forgot about ephemeral batches. LGTM

[nvanbenschoten]

bors r=danhhz

[craig]

Build succeeded

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