Add VELOC_Backend_ready()

[Matthew-Whitlock]

Adds ability to query if asynchronous checkpoint has been completed by the VeloC backend server, returns true if in synchronous mode.

[Matthew-Whitlock]

It would especially be worth double checking the changes to client.cpp lines 35-39, though I think they're right.

[bnicolae]

@Matthew-Whitlock: Thanks for the commit! Would you mind some minor adjustments?

• let's call it checkpoint_finished(), in line with the rest of the API calls
• the check_completion() condition needs to be protected by a lock: scoped_lock cond_lock(data->mutex);
• there are multiple control planes alternatives available in addition to ipc_queue, but I can add the missing capability later
• style: don't use brackets for single instruction blocks in if..then..else, break "if (condition) return" into two separate lines

[Matthew-Whitlock]

These changes are made now, thanks for the review!