With proper DN escaping.

Meant to integrate well with DNS-based service discovery: the client
will constantly monitor the DNS results for changes, and dynamically
modify the list of memcache backends.

Replay protection

Closes #4

See merge request !1

This makes it possible to run the auth-server more easily for smaller
projects.

Add the related documentation to the README. Include a nullU2FStorage
handler to prevent nil pointer dereferences in U2F code.

Backed by memcache, like the U2F challenge storage. The code has been
refactored to abstract the common replicated memcache client away.

Plus a couple of lint style fixes.

Backed by one or more replicated memcached instances.

Introduces the *services_dir* configuration parameter.

Uses the clientutil.Backend abstraction for the user-meta-server
client API.

Useful for partitioned (sharded) services that need to make
partition-related decisions at authentication time.