Don't write MAM messages into an Mnesia archive if the size of the table
comes close to the 2 GB limit for tables with disc-only copies. That
way, the table is at least not corrupted when the limit is reached.
Let mod_mam_mnesia use transactions when storing or deleting messages.
If old messages of a user are to be removed, delete the user's archive
and rewrite it from scratch, as that seems to be much faster than
removing individual records with delete_object/1.
Closes#1065.