This is an ugly patch, sorry. It adds the following features to
rpc-test, to help with (stress) testing the Haskell RPC client:

- customisable repeat count for the RPCs
- customisable parallelisation factor
- options to show timing stats and other information
- capability to execute any RPCs already defined, as opposed to having
  the RPC hardcoded; this requires a data file (defaulting to
  "rpc.json") that has the serialised RPC call

The polymorphism/way the RPC library is implemented requires us to
have some duplicate code (operating on different types); I wasn't able
to found a way to make this more abstract.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>