Integrate any.sender to bulk send messages

[robdefeo]

Is your feature request related to a problem? Please describe. As part of eth-London hackathon Mailchain will integrate sending bulk transactions

Describe the solution you'd like

• [X] create config for any.sender
• [x] create any.sender signer
• [x] create any.sender sender
• [x] remove hash from sign method
• [x] modify keystore to return different sender based on type

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. The resources of the Mailchain team are limited, and so we are asking for your help. If this is a bug and you can still reproduce this error on the master branch, please reply in order to keep the issue open. If this is a feature request, and you feel that it is still relevant and valuable, please reply in order to keep the issue open. This issue will automatically be closed in the near future if no further activity occurs. Thank you for all your contributions.