storepool: reduce getStoreListFromIDs allocations

[shreyasganesh0]

getStoreListFromIDs builds a StoreList and a ThrottledStoreReasons slice on every replication and rebalancing decision, growing both result slices via append from a nil start. On clusters with many stores this repeated reallocate-and-copy was observed in #151952 to allocate ~1GB over a 5s allocation profile, adding GC pressure that can hurt foreground latency.

Both slices hold at most one entry per input store ID, so this pre-allocates them to len(storeIDs) capacity. A new BenchmarkGetStoreListFromIDs (50 stores) shows the improvement (same machine):

name                  old           new
GetStoreListFromIDs   81732 B/op    33711 B/op
                      8 allocs/op   3 allocs/op
                      23757 ns/op   12631 ns/op

Epic: none
Fixes: #151952

[blathers-crl]

Thank you for contributing to CockroachDB. Please ensure you have followed the guidelines for creating a PR.

My owl senses detect your PR is good for review. Please keep an eye out for any test failures in CI.

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