Add option to skip Tor send attempts in TorConfig

[yeastplume]

Addresses #451. This is really the only non-breaking way to implement this (anything else would mean adding parameters to the receive_tx foreign API function, which would break backwards compatibility.

• Adds a skip_send_attempt field to the TorConfig struct. If set to Some(true), the slatepack workflow will skip the Tor send attempt.
• To turn off TOR sends, via the API's, use the set_tor_config API function (included in both the Owner and Foreign APIs), setting skip_send_attempt to true
• In addition, all command-line functions that can send via Tor now take a --manual, -m flag. If present, the Tor send attempt will be skipped.