InstrManager : Check that the plugin requesting the use of a profile is a declared user.
Measure plugin is now a registered instrument user and release profiles after the end of a measure.
Fixing LoopTask database handling to avoid issues with timing.

Previously the database was not updated on start-up leading to an issue if a measure was loaded with the timing activated.
Fix monitor behavior with respect to multiple measures run in a row.

Previously on closing the MeasureSpy was requesting the monitoring thread to exit as did the engine which resulted in two (None, None) being put in the queue but only one removed. Now the upon exit the monitor simply signal it is dying but does not kill the thread.