ArangoDB fallback config support

[wajda]

Background

At the moment, Spline presumes that the database will be a single instance.
Would it be possible for it to handle a multi-node non-sharded cluster?

Feature

E.g. instead of passing a single endpoint, multiple endpoints could be passed.
These endpoints will return the current leader by querying http://IP_ADDRESS:PORT/_api/cluster/endpoints, returning json with the leader being the first in a list of endpoints.
In this case if the leader went down, another endpoint could be queried for the new leader.

(By @hamc17)

[wajda]

https://www.arangodb.com/docs/stable/drivers/java-reference-setup.html#fallback-hosts

https://www.arangodb.com/2017/12/introducing-the-new-arangodb-java-driver-load-balancing/