storage: avoid deadlock during merges

Merges could deadlock if the lease on the right-hand range was
transferred or renewed during the merge transaction and the new
leaseholder received any request before the GetSnapshotForMerge request.
See cockroachdb#27442 and the comments within for details. Avoid the deadlock by
moving the responsibility for waiting for a merge to complete from the
replica to the store.

Add a unit test that runs several get requests concurrently with several
merges to reliably test for regressions. The problem previously only
presented when stressing TestStoreRangeMergeWithData.

Fix cockroachdb#27442.

Release note: None

pkg/roachpb/errors.go

@@ -706,3 +706,13 @@ func (e *IntentMissingError) message(_ *Error) string {
 }
 
 var _ ErrorDetailInterface = &IntentMissingError{}
+
+func (e *MergeInProgressError) Error() string {
+	return e.message(nil)
+}
+
+func (e *MergeInProgressError) message(_ *Error) string {
+	return "merge in progress"
+}
+
+var _ ErrorDetailInterface = &MergeInProgressError{}