gocached: bound WAL size and expose db/wal size gauges

[bradfitz]

One of our cigocached servers' WAL had grown to 46 GB over ~87 days of
uptime, with walFindFrame pegging CPU and stalling go-cacher clients on
the CI runners. A CI vet job that normally runs in ~100s on a laptop
was sitting at 30+ minutes.

SQLite's autocheckpoint runs only PASSIVE checkpoints, which reuse WAL
space in place but never shrink the file on disk.

Fix:

• Set journal_size_limit=1 GiB per connection so even passive
  checkpoints will truncate down to that bound when they can.
• Run wal_checkpoint(TRUNCATE) once at startup (this is what shrinks
  the existing 46 GB WAL on the running server after deploy) and
  periodically (every minute) from a background goroutine, logging
  any partial checkpoints that hint at a long-running reader.
• Run a final TRUNCATE checkpoint on Close.
• Bump modernc.org/sqlite v1.45.0 -> v1.51.0 for general fixes
  accumulated since February.

Also add two new gauges for ongoing visibility:

gocached_sqlite_data_bytes  size of the main .db file
gocached_sqlite_wal_bytes   size of the .db-wal file

Updates tailscale/corp#42670

[bradfitz]

cc @tomhjp for post-submit review

[tomhjp]

Thanks for documenting everything inline, I learnt a lot reviewing this. LGTM