Unify configuration of short-term storage

Add the related documentation to the README. Include a nullU2FStorage
handler to prevent nil pointer dereferences in U2F code.
1 job for replay-protection in 11 seconds (queued for 1 second)
Name Stage Failure
test Test
--- PASS: TestProtocol_SerializeRequest (0.00s)
=== RUN TestProtocol_SerializeResponse
--- PASS: TestProtocol_SerializeResponse (0.00s)
ok git.autistici.org/id/auth 0.020s
# git.autistici.org/id/auth/server
server/authserver.go:398:33: cannot convert nil to type int
FAIL git.autistici.org/id/auth/server [build failed]
ERROR: Job failed: exit code 1