PushTxn returns the persisted transaction record, if it exists, as
part of its reply. When the pushee's TxnMeta in the pusher's request
is from a newer epoch, PushTxn advances only the Epoch of the returned
transaction, leaving the IgnoredSeqNums from the _previous_ epoch.

If the pusher then attempts to resolve the intent using the returned
transaction record, it may erroneously remove a write based on the
incorrect IgnoredSeqNums.

Resolves: cockroachdb/cockroach#175819
Epic: none

Release note (bug fix): Fixed a bug where a committed transaction
could silently lose writes. This could happen when (1) a transaction
rolled back to a savepoint before its first heartbeat, (2) was then
restarted at a higher epoch, and (3) was then successfully pushed and
had one of its writes resolved by a contending transaction. An example
of a transaction that would satisfy (1) and (2) is an explicit READ
COMMITTED transaction whose first statement is a write or locking
read that hits more than max_retries_for_read_committed retriable
errors and executes for more than 1 second.