blocks: correcting use of mutexes in file_meta_sink.

the gr::block class has a d_setlock mutex that's appropriate for this use instead of creating another for the child class.
qtgui: fixed a bug in the constellation plots that didn't allow us to reduce the number of samples.
qtgui: replaced use of local mutex with d_setlock from gr::block.

Partial response to issue #706.
qtgui: got a little carried away removing local mutexes. SpectrumGUIClass is not a gr::block and so doesn't have a setlock.
uhd: fixed tagged stream block gap handling in usrp_sink