add elapsed request time to request.log

Our request logs log by default using the 'common' pattern:

%h %l %u [%t] "%r" %s %b

The %t field prints the response time instant. Example:

127.0.0.1 - - [29/Jun/2015:12:32:15 -0300] "GET /nexus/service/local/lvo/nexus-pro/2.11.4-SNAPSHOT?_dc=1435591934977 HTTP/1.1" 404 657

When correlating when the request is first received ( useful when coordinating requests with builds or other external scheduled jobs, it would be helpful to determine the elapsed time of the request from the request logs. This can be accomplished by using this pattern in logback-access.xml:

%h %l %u [%t] "%r" %s %b %elapsedTime

We should adjust the default request log pattern to include elapsed time.