SecretStore: 'broadcast' decryption session

[svyatonik]

This is a (one of) preliminary PR for adding encryption/decryption support to SS service contract. Besides checking this PR, please try to read && criticize my approach.

So there exists Document key shadow retrieval session, which is currently used to read document key without reconstructing it on any Key Server. When used via HTTP API, it returns three items: 64-bytes decrypted_secret, 64-bytes common_point and an array of ECIES-encrypted EC scalars (each is 145 bytes). Each entry in array is generated by one KS and there are t+1 entries, where t is the key threshold.

Previously this session had only gathered all required data on single key server (the one which was asked to restore the document key). In service contract PR there's another approach - we need to confirm decryption by every participating key server. So this PR is about this - in special mode (when is_broadcast_session == true) all data is broadcasted by every KS && at the end of session it is known by every consensus group node.

Life after this PR: the idea is that every KS will call a service contract method, which in turn will raise a big event like this: event RestoredByKeyServer(bytes32 keyId, uint threshold, 64-bytes blob, 64-bytes blob, 145-bytes blob);. Yet the number of events could also be huge (if we're dealing with a big SecretStore and key threshold is big). The requester should wait for enough events and pass events data to secretstore_shadowDecrypt.

So the question is - do you think if this approach (described in previous section) is viable, or not? If not, we can now start with Document key retrieval session which is not as secure, as previous (document key will be restored on one of key servers before encrypting and publishing via log events). And later try to find a way to generate less spam (maybe will finally find a way to use commutative encryption in SS).

[5chdn]

Assuming approved by @debris :)