refstore: implement data retention policy

[dmke]

Currently, texd saves files sent with application/x.texd; ref=store content type on disk, but never deletes them.

Users should be able to define a retention policy using the DSN. For the dir adapter, DSN parameters could look like this:

• dir://./refs?keep=7d - to keep files for (at least) 7d before deleting

Since a file's atime attribute is unreliable (nowadays, Linux file systems are mounted with noatime option), we need to either update the reference's mtime on access, or come up with an alternative.

@corny suggested to keep an in-memory map (from reference hash to last-access timestamp), and prime it on startup with the mtime of existing reference files. I like that idea.

[dmke]

Current plan is to use a generic retention-policy query parameter, since this is not specific to the dir adapter (any adapter is free to ignore or err when encountering these parameters, though).

Possible values:

• retention-policy=keep - do nothing (the default, at least for dir adapter)
• retention-policy=purge-on-start - clears files on start-up
• retention-policy=access - use an access list to delete least used files, with the following limits:
  • access-quota=<bytes> - let cache grow up to <bytes> in size; allow common suffixes (100m = 100×1024×1024)
  • access-max-items=<num> - keep the most used <num> files
  • both limits default to "infinite", but at least one must be configured