Better couch_log performance

Building the messages to write to the console and the log file can be done outside the couch_log gen_event. This significantly increases the parallelism when many processes log messages. The following relaximation test graph shows a throughput increase using the attached patch against current trunk:

http://graphs.mikeal.couchone.com/#/graph/0379dbdaef29b1c0fbf03421540243f7