Change internal RPC client body values

Currently, all RPC payloads sent by the client to the remote node
daemons must be identical, due to how the data is passed
internally. This is deficient in both use (from the programmer's point
of view) and from the network traffic (cluster verify/disk data
gathering has a total payload which is O(n²) in size of the nodes
being queried, instead of O(n)).

This patch changes the RPC internals so that we always pass
dictionaries indexed by target node name. For the default use case,
when the payload is identical, we only serialise the payload once, so
the extra overhead is just a dict with the node names and values all
pointing to the same object. For different payloads, we will encode
the body multiple times, but hopefully the bodies will be smaller.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>