Add an improvement to log-mediator

[shafreenAnfar]

Consider the below Proxy service in which I have configured the log-mediator to print logs only when the category is set to DEBUG. However, there is no way to start the server with log-mediator debug logs enabled. 

Only way to print the property values inside the log mediator is by configuring debug mode for the log -mediator class in the log4j.properties. But this is wrong because aforementioned configuration is there for the implementer of log-mediator but not for the synapse user.

 <proxy xmlns="http://ws.apache.org/ns/synapse"
       name="test"
       transports="http,https"
       statistics="disable"
       trace="disable"
       startOnLoad="true">
   <target>
      <inSequence>
         <log category="DEBUG">
            <property name="someproperty" value="somevalue"/>
         </log>
         <respond/>
      </inSequence>
   </target>
   <description/>
</proxy>

Above issue is fixed with this PR.

With this fix synapse users can start the server as follows which will enable debug mood for log-mediator.

sh synapse.sh -synapseDebug

Following is a sample output of the log mediator.

2020-03-04 21:01:02,663 [-] [PassThroughMessageProcessor-1]  INFO LogMediator To: /services/test, MessageID: urn:uuid:13cc5420-1282-456b-a0c0-fcfa6457c28e, Direction: request, someproperty = somevalue