Summary: This new mode provides the capability of pushing to
multiple clusters with different number of nodes and different
partition assignments.

Compatibility: Although this new mode only works if both the BnP
job and the Voldemort servers are upgraded, the change can be rolled
out gradually without breaking anything. There is a negotiation
phase at the beginning of the BnP job which determines if all
servers of all clusters are capable and willing (i.e.: configured)
of using the new mode. If not all servers are upgraded and enabled,
then the BnP job falls back to its old behavior. Likewise, if a
server gets a fetch request from a non-upgraded BnP job, it will
work just like before. By default, servers answer the negotiation
by saying they support the new mode. The old behavior can be forced
with the following server-side configuration:

readonly.build.primary.replicas.only=false

Running in this new mode has several implications:

1. When running in the new mode, store files are stored in the
   BnP output directory under nested partition directories, rather
   than in nested node directories.
2. The MR job uses half as many reducers and half as much shuffle
   bandwidth compared to before.
3. The meta checksum is now done per partition, rather than per node.
4. Instead of having one .metadata file per partition, there is now
   only a single full-store.metadata file at the root of the output
   directory.
5. The server-side HdfsFetcher code inspects the metadata file and
   determines if it should operate in 'build.primary.replicas.only'
   mode or not. If yes, then the server determines which partitions
   it needs to fetch on its own, rather than relying on what the BnP
   job placed in a node-specific directory.
6. The replica type number contained in Read-Only V2 file names is
   now useless, but we are keeping it in there just to avoid
   unnecessary changes.
7. When initializing a Read-Only V2 store directory, the server now
   looks for files named with the incorrect replica type, and if it
   finds any, it renames them to the replica type expected by this
   server.

Other changes:

1. Added socket port to Node's toString functions. Also made the
   output of the Node's toString(), briefToString() and getStateString()
   functions more consistent.
2. Introduced new Protobuf message for the GetConfig admin request.
   This new message is intended to be a generic way to retrieve any
   of server config.
3. Refactored VoldemortConfig to provide access to any config by its
   string key. Also cleaned up a lot of hard-coded strings, which are
   constants now.
4. Various minor refactorings in BnP code.