cdx: move perms related handling to pywb.perms package, support
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
add new perms_handler for supporting direct permissions api
currently just returning ["allow"] or ["block"] for a single url
url-agnostic revisit testing!
add sample warc and cdx for url-agnostic revisits
add unit test and integration test
resolvingloader: pass callback instead of full cdx server
for use for loading cdx in case of url-agnostic revisit