[NB] Fix rpc_proxy...

[NB] Fix `RpcProxy` class.  Prior to this commit, all handler methods were
erroneously referencing the last command added.  To fix this, the callbacks are
wrapped using a partial function with the command as the first argument, which
is then saved in the partial function, causing the correct command to be used
in future callbacks.

Add `partial` function wrapper to coffee_helpers.
[NB] Update manager comparator: use new RPC proxy