## Problem
If for some reasons we already garbage-collected the data under an LSN
but the caller uses a past LSN for the find_time_cutoff function, now we
will report a missing key error and GC will never proceed.
Note that missing key error can also happen if the key is really missing
(i.e., during the past offload incidents)
## Summary of changes
Make sure GC proceeds by bumping the LSN. When time_cutoff=None, we will
not increase the time_cutoff (it will be set to latest_gc_cutoff). If we
really need to bump the GC LSN for maintenance purpose, we need a
separate API to do that.
Signed-off-by: Alex Chi Z <chi@neon.tech>
640ac4fc9e