This patch introduces a distributed system keyspace, used to hold
system tables that need to be replicated across a set of replicas
(that is, can't use the LocalStrategy).
In following patches, we will use this keyspace to hold a table
containing view building status updates for each node, used to support
range movements and a new nodetool command.
Fixes#3237
Signed-off-by: Duarte Nunes <duarte@scylladb.com>
