Make hardcoded settings configurable

[jtuple]

There are a few hardcoded settings in riak_ensemble that should be configurable. Examples include the leader tick rate, the follower timeout, the number of leader request workers, the backend alive tokens, etc.

These should be configurable via app.config. We should also evaluate the current defaults and determine if they are appropriate or not. For example, the 200 ms tick rate seems too aggressive for production use. Perhaps 1s, 2s, 5s, or even 10s is a better value.

/cc basho/riak#536

[jtuple]

Fixed by #28