memento support enabled by default, togglable via 'enable_memento' config property
supporting timegate and memento apis, no timemap yet
supporting pattern 2.3 for archival and pattern 1.3 for proxy modes
also:
simplify exception hierarchy a bit more, move down to utils
make WbRequest and WbResponse extensible with mixins (eg for memento)
custom processing ops, of which perms is a specific type
add lazy_ops test to ensure all cdx processing ops are lazy
perms: set up a 'perms policy' factory and perms policy implementation
perms policy setting results in a custom processing op
update tests to work with new config
IndexReader handles both cdx server + perms policy