Fixed route graph splitting and diagram mapping race condition problem

RouteGraphConnectionSplitter.doSplit now always splits the connection so
that part #1 is always the part that stays with the existing diagram
connection and part #2 contains the entities that are moved to the newly
created route graph connection resource. Part #1 is the part where the
"output terminal" attached to the connection lies. This simplifies and
clarifies the implementation of doSplit and does not require moving
diagram mapping statements around.

Also, more importantly RouteGraphConnectionSplitter and FlagUtil changes
ensure that diagram mapping is *not* executed in FlagUtil.join, which
was the main cause of the previous corruption. Diagram mapping is only
activated once for the affected diagram(s) after everything else is
done.

gitlab #549
gitlab #586

Change-Id: Icf7479e8e111bf663f637d6909419267cfa4eec4