storage: Txn GC check uses wrong txn

[bdarnell]

The recently-added check to ensure that we don't recreate a transaction that is old enough to be GC'd uses the wrong transaction (txn is used to read the existing txn record from disk, and here it is definitely empty because we handled existing txns in the preceding switch statement. Because it is empty, its timestamps are zero, always less than the GC cutoff. It should be looking at the request transaction, or the copy cloned into reply.Txn instead).

This means that after a transaction GC has occurred, no more transactions can be executed on this range. This is the cause of the current lack of progress with the photos app, and suggests a significant gap in our test coverage.

@cockroachdb/stability

[tbg]

Yikes. Will clean up after myself.

[tbg]

Fixed in #9524.