Implement migration code for the stake contract optimisation

[miloszm]

Summary

As per issue #1447, stake contract needs to be optimised. In order for this optimisation to take effect, the contract needs to be migrated, as otherwise stake data would be lost.

Possible solution design or implementation

Before a block is finalised, the block height should be checked against some assumed height, and if it matches, say block height is 20000, a migration is performed, using the migrate method of Session object.

Additional context

We need to take care of the ownership of the new contract - see Piecrust issue 336

[HDauven]

Resolved by #1464