FCPPluginClient.sendSynchronous(): Add assert()s to prevent poor choice
of timeout.

- I've done a test run of test.writeLock().newCondition().await() and it
will NOT throw upon negative timeouts, so we need an assert for that as
well.
FCPPluginClient.sendSynchronous(): Initial implementation

- Not complete. What's not complete has been added as FIXME, the
following commits will resolve it.