Support replacing/cancelling transactions

[fjarri]

There are some seeds of that currently, in the form of broadcast_transfer()/wait_for_transaction_receipt(). Off the top of my head, a general scheme can be:

• broadcast_transfer()/deploy()/transact() returning a PendingTransfer/etc object with all the necessary data (tx hash, nonce, the tx itself).
• Methods operating with pending objects for replacement/cancellation/waiting for completion.