JGRP-1774 Memory allocation improvements about Long encoding

It seems like encoding of _long_ s is a quite hot operation in JGroups; this small refactoring is aiming at removing some small buffer allocations.

https://issues.jboss.org/browse/JGRP-1774

Also included
- some descriptions in the ANT file to serve as reminders for "ant -p" users
- extracting the Long utility methods from Utils to NumberEncoding

I didn't change any of the protocols, the public APIs nor the actual encoding strategy so to make this backwards compatible.