Refactor vpp-japi

vlib does not allow concurrent connections, hence make sure the Java API
does not allow the user to instantiate them.

Also hide native methods from direct user invocation behind defensive
methods also detect if a particular connection has been terminated.
Disconnect is hidden befind standard close() method, as specified by
AutoCloseable.

Change-Id: Ib5079200ae4216cad84358a2174a41e90271a30b
Signed-off-by: Robert Varga <nite@hq.sk>