Proxy SIGTERM to nested auraeds

[future-highway]

Added a SIGTERM signal handler that will broadcast to all cells.

Notes:

• The Cells cache is no longer on CellService so that we can access it anywhere in auraed (including from the signal handlers).
• Drop on Cell will result in a SIGKILL to nested_auraed when the Cell is still in the Allocated state. I don't expect this to be called (except in tests), but it gives us some more assurance that resources will be cleaned up.

Future work (in a different PR) will be to make this all async.

RE: #199