The backup code keys per-server state by host:port. WAL file names
contain the lowercased hostname, while ServerName#getAddress keeps the
hostname's original case. For a region server whose hostname contains a
capital letter, the backup code therefore tracked the server under two
keys, and lookups between them missed. Incremental backups then fell
back to the oldest boundary of any server, so they could include WALs
that an earlier backup already covered. Replaying those WALs could
bring back deleted data, or fail the restore when the WALs held edits
for a column family that was dropped since. Boundaries were also stored
under the wrong key for later backups. The log cleaner missed the
host's boundary and kept WALs longer than needed. Various tests were
broken when running on a host with a capital letter.

BackupUtils#toHostKey now lowercases the key. The WAL path parsers, the
log roll (both variants) and the live-server check of the full backup
use it. Reads from the backup system table and BackupBoundaries also
normalize, because existing tables can hold mixed-case keys. On a
collision, the newest roll result and the oldest boundary are kept.

Mixed-case keys that are already stored are phased out as follows:
- The per-table boundaries (trslm rows) are rewritten with lowercase
  keys by the next backup that includes the table. A table that is not
  backed up again keeps its old row. Backups with continuous backup
  enabled do not rewrite this row.
- The roll results (rslogts rows) are never phased out, because nothing
  deletes these rows. Once the host rolls its WALs on the new code, its
  lowercase row holds the larger value, and the read keeps that value.
- The boundaries copied into each backup's BackupInfo keep their
  mixed-case keys until that backup is deleted. BackupBoundaries
  normalizes them.

The legacy log roll procedure looked up the previous roll result by
hostname without port, so that lookup never matched. It now uses the
full key.

TestBackupOfflineRS now runs with a mixed-case region server hostname,
so it covers this on hosts with lowercase names too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>