i1722-avoid-wallet-rescan-after-init

[pragmaxim]

Closes https://github.com/ergoplatform/ergo/issues/1722

Introduction of WalletPhase into state is necessary, I reverted a few alternative approaches but in the end I think that it must be decided in case ScanOnChain(newBlock) whether to rescan or not and for that we need WalletPhase information kept in state.