Move logger configuration from extension to explicit logger config section

Loading the file logger as an extension at runtime adds some unnecessary complexity to our code. One way to remove this abstraction would be to have ns_server add the logger configuration as a separate json object inside memcached.json. The object would look like:

"logger": {"filename": "logs/n_1/memcached.log", 

               "cyclesize": 10485760,

               "sleeptime": 19}

The settings inside "logger" can then be used inside memcached_main to configure the file logger at compile time.