Make it possible to tune the deactivation of grains in relation to memory usage

[COCPORN]

Make it possible to tune the deactivation of grains in relation to memory usage, both for the GC-reported usage and host environment. Orleans implements IHostEnvironmentStatistics and IAppEnvironmentStatistics and these can be queried against configuration to see if deactivation of grains should be suppressed.

The implementation should have functional parity with the current implementation to make any configurable new behavior not come off as surprising.

[ghost]

We've moved this issue to the Backlog. This means that it is not going to be worked on for the coming release. We review items in the backlog at the end of each milestone/release and depending on the team's priority we may reconsider this issue for the following milestone.

[ReubenBond]

Closing in favor of the existing issue, #370