Revisit CQ management

[jdinan]

In #785 we merged the CNTR and CQ EPs in an OpenSHMEM context. We should revisit CQ management:

• Quiet no longer needs to drain the CQ (it's sufficient to wait on the CNTR)
• Having the CQ handy makes it easier to generate individual completions for blocking operations (i.e. to use the completion flag that's passed down to the transport layer).