Abstract a function to get a logger implementation expression.

It handles atoms in compile time and falls back to run-time call to
ale_utils:logger_impl in other cases.

Change-Id: I6bd965fe562afee941fe6e7843f3dd1b8a199d5c
Reviewed-on: http://review.couchbase.org/37518
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>