FCPClientReferenceImplementation: Fix bad thread synchronization

- Symptoms would be IllegalMonitorStateException, but also general lack of synchronization upon FCPClientReferenceImplementation member variables.
- In general, what was wrong was that I forgot that I was writing a member class of FCPClientReferenceImplementation, so I used threading stuff on the member classes instead of FCPClientReferenceImplementation.this
- (Indentation fix will not be committed since this has to be merged
into use-plugin-fcp-rewrite, and that branch will soon replace a lot of
this class, so we do the indentation there.)