Support record signatures on blockchain

Smart contracts v0.5.x are supporting the additional signature field: pheix-research/smart-contracts@74142c92. We have to support it too in Pheix.

Full details: #176 (comment 1411809341)

This is RC2 feature and requires straight-forward changes on Pheix side. The principal thing: it should be configurable on application level at least.

Coverage by unit tests, requested at #176 (closed), will be done transparently — we can just switch on signature checks for one of blockchain tests (for example for /t/07-blockchain-common.t) and it should work out of the box.

Signatures are stored and validates automatically:

1. Storing by row_insert and update methods
2. Validation by select and get_data_byindex methods