Some plugins, such as the 'AGain' demo from the VST3 SDK and Diginoiz
Subdivine segfault when setParamNormalized is called from a background
thread.

This patch attempts to ensure that calls to get/set parameter values can
be made safely from the audio or message threads.

It also adds some MessageManagerLocks around some other calls to the
EditController's member functions which should technically only be made
from the GUI thread.

Finally, it updates the parameter input queues to the IAudioProcessor so
that writing to the queue is wait-free. As part of this change, a new
queue type is introduced which holds up to a single point. This should
improve memory usage for plugins with many parameters.