Cleanup OvsdbDataTreeChangeListener

Cleanup OvsdbDataTreeChangeListener

This patch restructures the logic a bit, so that it does not invoke
getData{After,Before}() multiple times, fixing Eclipse warnings and
improving performance.

Furthermore augmentation handling is refactored so that the two
cases (node/bridge) are handled in an if cascade, preserving type
safety.

Finally updateConnections() is refactored to use a switch statement
instead of multiple if conditions, which shows that delete of the
entire node are not handled -- a FIXME is placed there.

Change-Id: If5663ebd2944d7fe83c4928b96cedc0db0f90e19
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>