Bug 9755: Refactor record merge functionality

This patch refactors the merge record interface and code a little bit
in preparation for making it possible to merge authority records.

To test:
1) Apply patch.
2) Try merging two records:
    a) Create a list.
    b) Add two records you would like to (or be willing to) merge
       to said list.
    c) View said list.
    d) Check the checkboxes next to the two records you added.
    e) Click "Merge selected records."
    f) Choose a merge reference.
    g) Choose fields from each record that you want to keep.
    h) Click "Merge."
3) Confirm that your merged record has the fields and subfields you
   wanted.
4) Run the unit tests for the two files that were changed:
    prove t/Koha_Record.t t/db_dependent/Koha_Authority.t
5) Sign off.