Switch the RPC module over to the multi interface

This replaces the very-basic parMap of IO actions (fully serialised,
as parMap won't work here), to the multi interface.

This makes a simple "time gnt-node list" on a 6-node cluster go from
3.2s to ~0.9s, and allows even better parallelisation - before,
curlGetString was blocking, whereas the new interface does allow some
interleaving.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>