Change set/remove to require a lock for the branch config files.
This means that tests (or any plugin for that matter) do not requires an explicit lock on the branch anymore to change a single option. This also means the optimisation becomes "opt-in" and as such won't be as spectacular as it may be and/or harder to get right (nothing fails anymore).
This reduces the diff by ~300 lines.
Code/tests that were updating more than one config option is still taking a lock to at least avoid some IOs and demonstrate the benefits through the decreased number of hpss calls.
The duplication between BranchStack and BranchOnlyStack will be removed once the same sharing is in place for local config files, at which point the Stack class itself may be able to host the changes.