kvserver: reuse memory for roachpb.Key in latches and abort span (>1% allocs on oltp_write_only)

We allocate ad-hoc keys for abort spans and transaction related keys on hot paths. Many of them seem to share a lifetime with the `SpanSet` latches. There is already a pooling facility for span sets and I don't think we routinely leak these created keys elsewhere, so it's possible that we can avoid most of these allocations by creating these keys from a pool associated with span sets and returning them when the spanset is finished.

Epic CRDB-55523
Jira issue: CRDB-45640