Fix Pos() deadlock with file system invalidation

[benbjohnson]

This pull request fixes a deadlock that occurs between litefs.DB.mu and the FUSE file system invalidation. Currently, the FUSE library is set to be synchronous so holding the litefs.DB.mu lock while committing to the journal and reading the -pos file can cause a deadlock.

The fix is to remove the sync.Mutex from litefs.DB as it was no longer necessary because SQLite obtains locks for all the calls inside that alter data. The litefs.DB.pos field has been converted to an atomic.Value so that its value can be obtained independently of the SQLite locks. This is useful as a write transaction on the rollback journal would prevent inspection of the position.

Originally reported by @kentcdodds in this Fly.io community forum post: https://community.fly.io/t/site-falls-over-every-few-hours-before-rebooting/8907/37