Add explicit logger config section to memcached.json

Currently, when ns_server generates the settings in memcached.json, the file logger configuration is specified as an extension field. Eg:

"extensions": [

    {

       "module": "/Users/eugen-virtan/repos/couchbase/source/install/lib/memcached/file_logger.so",

       "config": "cyclesize=10485760;sleeptime=19;filename=logs/n_0/memcached.log"

    }

  ]

The use of an extension interface adds unnecessary complexity which can be removed by moving the logger configuration from the extensions field into a separate JSON object.

KV-Engine would like to move to a simpler; more explicit model where we configure the logger with it's own config group:

"logger": {"filename": "logs/n_0/memcached.log", 

               "cyclesize": 10485760,

               "sleeptime": 19}

The interface which parses this new "logger" object has been added in the following patch: http://review.couchbase.org/#/c/86015/

The logger object can contain the following fields (all have sensible default values if they are not specified):

• "filename": "string" -> the path to and the prefix of the logfile
• "buffersize": unsigned int -> the size of the logger's internal queue (in bytes) - note will be rounded by KV-Engine to a power of 2 (currently moving over to spdlog, which only supports powers of 2 for the queue size)
• "cyclesize": unsigned int -> max size of a logfile (in bytes)
• "sleeptime": unsigned int -> seconds between forced flushes of the log buffer
• "unit_test": bool -> if running in a unit test or production (ns_server can ignore this).