Whitespace cleanup.
Typos.
Change guard into pattern matching.
Unify `remove_user`

We expect only binaries to be handled.
Remove unused hook.
Simplifie call logic.
Simplify variables scope.
Bug Fix: changed to dirty read, since function is called outside of transaction.
Removed `optional` attribute from roster versioning.
Change JID to LJID.
Change JID to LJID when creatign roster not found in backend.
Use version which allows to count database reads

Preserve logic of commit b255dc27bf03742765a223081aea3304d317fafc
List to binaries, and removed whitespaces.
Cleaner version, and explicite handling of possible `not_found` roster version.
Change comment to possible future improvement.
Log if we fail to convert database entry to roster record
Dynamic selecting of backend module.
Removed unneded file with roster logic.
Renamed backend file to fit naming and dymanic loading convention.
Moved backend behaviour definition to `mod_roster` module.