Fix implementation of Relation.size.

Nothing was actually using it yet, fortunately.
Allow zinc to use InternedAnalysisFormats.
sbt-vaadin-plugin added to community plugins
Log socket accept errors when forking tests

If an exception is thrown when accepting a connection from a forked test
agent, currently I'm seeing that all that happens is SBT hangs with no
output.  Thread dumps show that the main process is waiting for the
agent to return, while the agent is waiting for the server to send it
something.

This change logs the exception, so that at least the error can be
googled.  It also cleans up the server socket.