[meshing] provide support to pause meshing during a session

[rcabanier]

From @thetuvix:

One pattern that is common on HoloLens is for an app to have a scanning phase and then a running phase. The user starts the app in a new room where the device hasn't been before. The app asks the user to look around until the mesh that's been scanned is sufficient for the app to do whatever analysis and feature isolation that it needs. At a minimum, we need to allow apps to scan for some period and then stop scanning.