Allow configuration of consistency policy

[kelvinmwinuka]

EchoVault is currently strongly consistent. This change will aim to make this policy configurable.

At the moment, echovault has a memberlist layer and a raft layer. The memberlist layer is mainly used to establish a cluster skeleton, essentially the "chassis" of the cluster. Memberlist nodes will communicate with other nodes to ask permission to join the raft cluster using the gossip protocol. Once the gossiped message reaches the raft cluster leader, the leader will add the node to the raft cluster.

We must allow a user to configure this functionality using the config flags. The option will enable the user to forgo the raft layer and use the memberlist layer for gossip-based state propagation to allow for eventual consistency. Otherwise, the user can opt for the strong consistency of the raft layer.