GAS and NEO tokens are sent to validators account (not the committee's
one). For single-node chain they are the same, but for four-nodes chain
they are different. Thus, use validators multisig address to create new
accounts and to deploy contracts.
Also, allow to provide desired account balance while creating new
account.
Implementing a separate `Signer` interface is beneficial in multiple
ways:
1. Support both single and multiple transaction witnesses.
2. It should be easy to add contract signer this way.
Tests should use accounts created with `NewAccount` so hiding all
details doesn't seem to be an issue.
Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>
