Add configurable limit to ensure a minimum amount of free space is preserved

- default to 100MB of free space
- currently disabled on android due to missing sys/statvfs.h
Cleanup the rhizome database every 30 minutes or so
Test that msp listen --service_name shuts down
Add java API wrapper for performing multiple config changes in one hit
Open the database immediately on config load
Merge latest MeshMS Java API additions into 'development'