fixup! Backport to ejabberd-2.1.x (strings everywhere)

Attempt to make mam_SUITE:prefs_set_request/1 pass.

I should really bite the bullet and switch to modern ejabberd :(

Radek spotted this one. Basic sloppiness on my part.
added_at is insufficient to guarantee ordering
Use DEBUG rather than INFO_MSG in most places

We still have an INFO_MSG for start/2, stop/1, and on_remove_user/2,
but creating multiple INFO_MSGs for every stanza that passes through
the system is a bit excessive.
s/handle_package/handle_packet/
Convert timestamps to milliseconds

This is to support the automated testing of naive clients that store and
work with server timestamps exclusively. Seconds-precision causes too
many causes for obvious re-ordering.

(Only checked by grepping for [^i][Ss]ec so far.
Automatic type checking might be useful at some point.)
rename table to mam_messages

This makes it feel more like the rosterusers table.