dcache,nfs,dcap,srm: Drop duplicate LoginStrategy cache

DCAP had its own simplified cache for LoginStrategy. This patch replaces
it with the generic implementation. The generic implementation is moved
out of the login service package, as the cache is unrelated to the login
service.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7329/