multiple "permissions" methods at once

[meejah]

Once the "permissions" implementation lands (#23) it would be useful to add the ability to have multiple implementations active at the same time.

For example, "none" and "hashcash" for now: this would allow clients to send hashcash even if they don't "have" to -- but also prepare for a third permissions implementation (when it might be actually-useful to have multiple methods turned on "at once").