Flush ObjectOutputStreams after construction

This protects against deadlocks between the writing and reading end,
since the ObjectOutputStream constructor writes a header, but does not
flush, and the ObjectInputStream constructor reads the header, and
blocks until it's read.
avoid deadlock in ConsoleOut.systemOutOverwrite

System.out can be reset after being captured by `val lockObject`.
Then locking `lockObject` and calling `println()` could lead to a
deadlock.
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.
update to ScalaCheck 1.11.0
Explicitly, optimistically close export streams early: workaround for #937

This is a temporary workaround: it assumes nothing else uses these streams later.
This condition is ok for 'export' and test streams, since these are unlikely to
reuse these streams.  However, the proper fix is for the TaskStreams methods
to be smarter- they could open in append mode if the stream was closed.  The
streams associated with a task could be optimistically closed after it finishes executing.
(Any task can write to another task's streams, which is why it is an optimization only.)
Continuation of previous commit's workaround for #937: close test streams early