OF-3044: When resource binding, ensure that old detached sessions are removed
After resource binding succeeds, clean-up of older, detached sessions will be problematic (as that needs to update state, but typically also sends out presence unavailable on behalf of the user).
In this commit, any detached sessions matching the full JID that will be the result of a successful resource binding is terminated, prior to resource binding being allowed to complete.
code cleanup: use isEmpty instead of numeric comparison
Replace `.size()` and `.length()` comparisons with `0` with a call to `.isEmpty()`, which is more descriptive and thus a bit less error prone.
