More new code for 5.13:

- Rename the log timestamp struct.
- Remove broken transaction counter debugging that wasn't working
  correctly on very old filesystems.
- Various fixes to make pre-lazysbcount filesystems work properly again.
- Fix a free space accounting problem where we neglected to consider
  free space btree blocks that track metadata reservation space when
  deciding whether or not to allow caller to reserve space for
  a metadata update.
- Fix incorrect pagecache clearing behavior during FUNSHARE ops.
- Don't allow log writes if the data device is readonly.
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