propagate timestamp to feed_tx.

pull VerifyError from script.
feed_tx: needs timestamp (to decided on script rules).
feed_block: pass block timestamp.
libsecp256k1 API has changed.
call secp256k1.start with flags.

verify: translate exception to 0 return.
address_to_key: checks version rather than returning it.

Also, simpler implementation.
ping-on-idle.

go: send a ping on an otherwise idle connection.
cmd_pong: check against the nonce last sent.