Kvs client-server

This release brings a new backend sled into the mix. You can now start KVS with either kvs which is the default or sled

kvs-server --engine sled --addr <Socket> 
#  Default socket is 127.0.0.1:4000

The client is what you'll use to talk to this server. Same defaults as kvs-server:

kvs-client --addr <server-socket> set hello world
kvs-client --addr <server-socket> get hello 
> world
# If using default port 4000, you can omit the `--addr` flag
kvs-client set awesome true