HF-26 transaction serialization support ( part of issue #291 )

It's possible to process a transaction signed using either legacy or hf26 way. Implemented support for switching transaction serialization at signature generation like also added 2-way authorization verification (if HF26 is active, first tries HF26 serialization mode, second lagacy one).

Transactions are still serialized using legacy way during p2p transmission and during block serialization. Further changes will be continued in next MRs