The changes in cockroachdb#33396 made it so that a `HeartbeatTxnRequest` that finds
a missing transaction record will attempt to create the record. This
means that it will discover if a transaction is not "committable" and
return a TransactionAbortedError. Unfortunately, after a transaction
commits and GCs its transaction record, it will also be considered not
"committable".

There will always be cases where a heartbeat request races with an
EndTransaction request and incorrectly considers the transaction
aborted (which is touched upon in a TODO a few lines down). However,
in many of these cases, the coordinator already knows that the transaction
is committed, so it doesn't need to attempt to roll back the transaction
and clean up its intents.

This commit checks for these cases and avoids sending useless rollbacks.
The intention is to backport this to 19.1.

Release note: None

This change increases the duration between transaction heartbeats
before a transaction is considered expired from 2 seconds to 5 seconds.
This has been found to dramatically reduce the frequency of transaction
aborts when a cluster is under significant load. This is not expected
to noticeably hurt cluster availability in the presence of dead nodes
because we already have availability loss on the order of 9 seconds due
to the epoch-based lease duration.

This is especially important now that the hack in cockroachdb#25034 is gone. That
hack was hiding some of this badness and giving transactions a bit more
room to avoid being aborted.

Release note: None