* Start work on passing context to backends
* More work on passing context
* Unindent logical system
* Unindent token store
* Unindent passthrough
* Unindent cubbyhole
* Fix tests
* use requestContext in rollback and expiration managers
A buffer is used to ensure that we only remove certificates that are
both expired and for which the buffer has past. Options allow removal
from revoked/ and/or certs/.