SecretStore: ability to identify requester via Public/Address

[svyatonik]

In client sessions via HTTP requester (the one who queries/creates the server/document key) is always 'identified' by passing signature of server key id as the parameter.
While working with service contract, we can determine requester' address from msg.sender and it seems counter-productive to ask for another identification data (another security-related reason: everyone could intercept/read the signature from transaction and use it in HTTP API if available). So in service contract we will use msg.sender where possible and ask for full sender' public when required. In details:

1. when generating server key: only msg.sender is required to remember in SS db and then authorize the same sender to store linked document key
2. when storing document key: only msg.sender is required
3. when querying document key AND when signing using server key: we also need the full public of msg.sender (to encrypt returned data). The contract will be responsible to check that address(public) = msg.sender

[folsen]

Looks pretty straightforward refactor to me, also really like the ability to have more ways to identify a requester.