Move Bitcoin SPV: initialization

[robert-zaremba]

To initialize a contract we need to set a "start" block: light block + combined difficulty, that will be the start (or a "snapshot") for the light client. It can be the genesis light block (height 1 or 0) or any block in the past.

The idea is to be able to start the LC from a recent block (eg yesterday) rather than from the genesis.