Fix PurgeEx replay with sequence & keep succeeds

[MauriceVanVeen]

PR #4212 fixed the issue I reported in #4196.

However, I believe there might be a bug when both sequence and keep are set during recovery.
In the PurgeEx the following check is done (for both filestore.go and memstore.go):

	if sequence > 1 && keep > 0 {
		return 0, ErrPurgeArgMismatch
	}

The TestJetStreamClusterPurgeExReplayAfterRestart also triggers this case, meaning that during the test this error is returned but it succeeds because the purge was already performed. Is this intended behaviour?

To elaborate a bit more, I believe the following happens:

• when running the purge normally it will properly run the keep (since it's not combined with sequence yet)
• when replaying the purge though, the sequence is added to the keep, which errors out in the above if

Which means that during normal operation all will be well, but purges with keep will be ignored upon replaying.

I'm proposing to remove the sequence > 1 && keep > 0 check and subsequent error. Which, for reference, was introduced in #3121.
Hoping this ensures that during recovery, purges that haven't executed yet will still be executed.

An alternative approach, which wouldn't remove the error: not allow combining sequence and keep normally and only allowing it during recovery. Which would preserve the current behaviour, and correctly apply sequence+keep during recovery still. However, not sure if it's possible to know if we're in "recovery mode" from within PurgeEx.

Resolves #4196

[derekcollison]

LGTM