optimize sequel store wrap_block
bitcoin_wallet: send to type "address" by default
block builder: allow to add already existing tx
tx builder: option to auto-create change output with fees
make #verify_input_signature work with passing only script_pubkey

... instead of the whole outpoint_tx
this enables SPV clients to verify signatures when they only know the
tx_hash/index and pk_script of the prev_out and not the whole tx.
allow builder to create txins with just the prev_hash and pk_script, instead of the whole tx

TODO: fix wallet specs
relay blocks to peers
add txout index to "monitor output" response message
more robust blk/tx/out monitoring

allows the client to specify the last object it knows about and sends everything the client has missed
fix monitor notifications on multiple channels interfering with each other
Tx#mutate to simulate tx malleability issue
change command API to more extensible json requests/responses
change monitor output API to return hashes with named keys, and add tx.nhash
storage notifications for main blocks and reorgs

TODO: side/orphan blocks, orphan reconnect, etc