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.
Databases and frames now require explicit creation and have the
option of setting row & column labels. If no labels are provided
then the default `id` and `profileID` labels are used.
This commit adds a `Closing` channel to the `IndexSyncer` and the
`FragmentSyncer` that is periodically checked during execution.
If the channel is closed then an in-progress sync is immediately
stopped.
