Introduce the arc_mod module

Here is the mod_mam module. It includes:
- a function, that escapes BLOBs;
- monitors for ODBC workers (they delete an entry from the sql_pool table, when a worker dies);
- the `ejabberd_users` module, that contains a cached version of the function `is_user_exists`;
- the `ejabberd_node_id` module, that assigns an unique integer (in a range from 0 to 255) for each node of the cluster;
- the `ejabberd_odbc:get_dedicated_connection/1` function, that creates a separate ODBC worker.

It does not include:
- the Riak backend for `mod_mam`;
- the pgSQL backend for ODBC;
- the poolboy integration.