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.
Sometimes user needs to construct transaction by itself, so it's better
to unify nonce sources for auto-generated and manually-generated
transactions to avoid nonce collisions in tests.
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>
