Changes made

This PR implements replication.

Added /replica endpoint to differentiate coordination from writes.

Changed Put/Delete in http_handlers.go to coordinate writing/deleting replicas.

Added PutReplica/DelReplica to differentiate write only steps that are done when targeting the /replica endpoint

PUT

• Builds a new PUT request that targets with the /replica endpoint.
• Put now uses ReplicasForKey instead of OwnerForKey (LookupN used internally instead of Lookup).
• PutReplica used by /replica endpoint to write the key/value

DELETE

• Builds a new DELETE request that targets with the /replica endpoint.
• Delete now uses ReplicasForKey instead of OwnerForKey (LookupN used internally instead of Lookup).
• DelReplica used by /replica endpoint to delete the key

GET

• Changes out of scope for this PR.
• Get uses OwnerForKey to look up the owner which always returns the next logical owner in the ring.
  • For example we have owner node A with a replica on node B. If node A crashes the gossip protocol marks it DEAD and it is removed it from every alive node's ring.
  • Next time OwnerForKey is called, the ring doesn't have A anymore, so it returns that B is the owner.

Fixes #1