Margo-based implementation

[mdorier]

This PR provides a margo-based implementation of the mercury-progressor. This implementation can be enabled by providing -DENABLE_MARGO=ON to cmake.

From an API perspective, existing codes can continue to use the exact same API. The use of Margo will simply change how the progress loop is managed internally. Enabling margo also provides two extra functions:

• mercury_progressor_init_from_margo(margo_instance_id mid) to initialize a progressor using an existing margo instances;
• mercury_progressor_mid(progressor_handle_t*) to get a margo instance from a progressor handle.

This PR also improves the existing test to add an RPC, and provides another test when margo is enabled, in which one RPC is done using Mercury and another is done concurrently using Margo, showing interoperability.

Contrary to the original implementation of the mercury progressor, the Margo-based implementation will keep the progress thread running all the time (the original implementation stopped the thread when it wasn't needed and respawned it when needed). This is something that could be fixed, if needed (we don't have a way right now to pause the progress loop in Margo, but we have an old PR with the feature, that was never merged and that we could revisit).