base: rework DataSource JNDI ObjectFactory

Don't try to serialize the full DataSource. It doesn't quite work
as the underlying c3p0 pool manager is not serialized, which means
that each lookup results in creating a new pool.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
Merge branch 'master' of github.com:killbill/killbill-platform