Separate out block_opened/closed() from TxOwner

We should separate out the block related methods from TxOwner in the following grounds:

1. Some modules not implementing any Tx may serve TxOwners and may want to have chances to prepare to serve them and/or clean up after serving all.
2. Modules implementing multiple types of Txes may get notified on block open/close only once. With TxOwners exported for all Tx types implemented by a module, all of the TxOwners will be redundantly notified.