Provide a "--local" flag to "bin/conduit call" to force in-process execution

Summary:
See PHI1692. Currently, it's hard to get a local profile or "--trace" of some Diffusion API methods, since they always proxy via HTTP -- even if the local node can serve the request.

This always-proxy behavior is intentional (so we always go down the same code path, to limit surprises) but inconvenient when debugging. Allow an operator to connect to a node which can serve a request and issue a --local call to force in-process execution.

This makes it straightforward to "--trace" or "--xprofile" the call.

Test Plan: Ran bin/conduit call ... with and without --local using a Diffusion method on a clustered repository. Without --local, saw proxy via HTTP. With --local, saw in-process execution.

Differential Revision: https://secure.phabricator.com/D21114