kv: initialize consistencyLimiter during Store construction, before Start

[nvanbenschoten]

Fixes #96231.

This commit attempts to fix #96231. It moves the initialization of Store.consistencyLimiter up from the bottom of Store.Start into NewStore. It is unsafe to initialize this variable after the call to Store.processRaft, which starts Raft processing. Beyond that point, incoming Raft traffic is permitted and calls to computeChecksumPostApply are possible.

The two stacktraces we have in that issue conclusively point to the Store.consistencyLimiter being nil during a call to (*Replica).computeChecksumPostApply. This startup-time race is the only possible explanation I could come up with.

Release note (bug fix): Fixed a rare startup race that could cause an invalid memory address or nil pointer dereference error.

[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?

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[cockroach-teamcity]

This change is 

[nvanbenschoten]

bors r=aliher1911

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

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