Expose statistics configuration attribute per JGroups protocol

By default, every JGroups protocol gathers statistics. Currently, this can be disabled per protocol via:
<property name="stats">false</property>

This is a bit cumbersome. Also, a standard property like this should be 1st class attribute of the <protocol/> element.
e.g..
<protocol type="..." statistics-enabled="false"/>

We may also want to add the ability to disable statistics per stack, so the user does not have to explicitly disable it in every protocol. That way, to enable statistics for a single protocol, one could use:

<stack name="udp" statistics-enabled="false"><!-- Disables collection of statistics for all protocols -->
<transport type="UDP" statistics-enabled="true"/><!-- Enables collection of statistics for just this protocol -->
...
</stack>