Support returning responses to users only after the transaction is committed

[achamayou]

CCF today returns responses as soon as they become available, before consensus has been achieved. It is then the responsibility of the user to check that they do commit eventually.

This isbeneficial on SGX where EPC/fast memory was scarce, and where max enclave sizes remain small due to startup times: freeing the response as quickly as possible is critical to keeping memory usage down.

On AMD hardware however, we could afford to keep responses on the node for longer, providing the user with the convenience of only seeing committed responses. It's worth noting that this increases somewhat the window of loss for responses as well.