Fix hang on error during Live, esp. with Multi Camera

Multiple race conditions fixed. The code is not ideally structured, but
good enough for now.

This prevents some freezes when Multi Camera is used with a circular
buffer that is too small (an error is reported instead of hanging).

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14519 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd
Refactor timer management in LiveModeTimer

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14520 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd