Remove the db.lock file per replica

[cloutiertyler]

Currently we write a db.lock file in the replica directory for each replica, however, this should really not be necessary since we already have a data-dir lockfile. This db.lock lock file has caused us issues with databases not being able open, so we would like to remove this file.

TODO:

• Investigate and ensure that this lockfile is truly redundant
• Ensure that opening the replica twice will not have problems
• Remove the lockfile.

[kim]

The db.lock file is a safeguard to not open multiple writers for the same replica, which is orthogonal to the data-dir lockfile that just protects against two processes operating on the same server data dir.

If a database refuses to start because there is a locked db.lock, then something is still holding this lock. So that's a bug and it is a good thing we have the db.lock, as otherwise we wouldn't notice.