Simplify validity check for loglevel expression.

Reject only atoms that are not one of the known loglevels.

Change-Id: Iba5c6b20b2a98864e5aad1512568d7b5839aba54
Reviewed-on: http://review.couchbase.org/37520
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
(Pseudo-)Functions to inspect effective logger's loglevel.

ale:get_effective_loglevel(Logger)
  returns effective loglevel for a logger

ale:is_loglevel_enabled(Logger, LogLevel)
  returns true if Logger has LogLevel enabled

Change-Id: Ic9f0bd0e28d407c1652bc8471b72d415dc202d71
Reviewed-on: http://review.couchbase.org/37521
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>