Changes Gossiper to NodeSet.
Adds StaticNodeSet (for testing) and GossipNodeSet (for memberlist) implementations.
Changes NodeSet interface to return a pilosa-specific generic instead of `NodeSet interface`.
Removes NumMembers from interface (which is specific to memberlist).
Implements a Messenger interface with which to send inter-node messages via NodeSet.
The Pilosa implementation occurs in the GossipNodeSet.
Implements the Messenger as an object on Server, Handler, and Index.
Uses HealthStatus constants.
Removes commented-out code.
for gossip, make sure to bind to both host and port, and advertise those as well
adjust messenger to work with the db schema logic
add dependencies: hashicorp/memberlist, golang.org/x/sync
add dependency: golang.org/x/net
Uses `errgroup` to handle errors from broadcast messages.
Marshals message one time instead of once for every node.
Adds error handling for some errors that were being swallowed.
