Make synctree LevelDB options user-configurable

This commit makes the LevelDB options used for ensemble synctrees
configurable in app.config.

This commit also uses the randomized write_buffer pattern used elsewhere
in Riak, where the LevelDB write buffer size is set to be a random value
within a given min/max range. Randomizing the write buffer per LevelDB
instance helps protect against multiple instances needing to flush their
write buffers at the same time.