allocator: add support for store pool liveness overrides

[AlexTalks]

While previously the allocator only evaluated using liveness obtained
from gossip, this change introduces a new OverrideStorePool struct
which can be used to override the liveness of a node for the purposes of
evaluating allocator actions and targets. This OverrideStorePool is
backed by an existing actual StorePool, which retains the majority of
its logic.

Depends on #91461.

Part of #91570.

Release note: None

[cockroach-teamcity]

This change is 

[kvoli]

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @AlexTalks and @andrewbaptist)

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pkg/kv/kvserver/allocator/storepool/override_store_pool.go line 29 at r1 (raw file):

// The OverrideStorePool's methods fall through to the underlying StorePool
// while passing in the configured NodeLivenessFunc to be used in place of the
// underlying StorePool's. This is only the case, however, for StorePool methods

nit: This last sentence is hard to parse - could it be reworded to be clearer?

Code quote:

// underlying StorePool's. This is only the case, however, for StorePool methods
// that are read-only, despite StorePool.DetailsMu being held in write mode in
// some functions, avoiding the possibility of mutating underlying state.

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pkg/kv/kvserver/allocator/storepool/override_store_pool_test.go line 179 at r1 (raw file):

	liveReplicas, deadReplicas := sp.LiveAndDeadReplicas(replicas, false /* includeSuspectAndDrainingStores */)
	if len(liveReplicas) != 5 {
		t.Fatalf("expected five live replicas, found %d (%v)", len(liveReplicas), liveReplicas)

Why not use the require lib in these tests?

[AlexTalks]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @andrewbaptist and @kvoli)

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pkg/kv/kvserver/allocator/storepool/override_store_pool_test.go line 179 at r1 (raw file):

Previously, kvoli (Austen) wrote…

Why not use the require lib in these tests?

Done.

[AlexTalks]

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)