Fix bugs in mod_mam_odbc_prefs

[arcusfelis]

Add tests for MAM preferences
Bug 1: JIDs are not normalized before stored in preferences
Bug 2: Result from SQL query never matches in get_behaviour

[arcusfelis]

Progress so far:

• More tests for MAM prefs.
• Fixed mam_*_prefs modules. None of them worked.
• Reduced mam_SUITE execution time.

[arcusfelis]

Ready for review.

[erszcz]

Let's not forget to remove apps/ejabberd/src/mod_mam_mnesia_prefs.erl and the relevant tests once this PR is merged. To make things simpler, let it be another PR.

The reason is that it duplicates functionality of mod_mam_mnesia_dirty_prefs while also probably being less efficient. All in all, MAM seems complex enough right now, we don't need more modules doing the same thing (as it was with mod_mam_user and mod_mam_server_user) and hyper-configurability to the degree of madness ;)