mem-cred: Support storing a delta CRL together with its base

mem-cred: Support storing a delta CRL together with its base

So far every "newer" CRL (higher serial or by date) replaced an existing
"older" CRL.  This meant that delta CRLs replaced an existing base CRL
and that base CRLs weren't added if a delta CRL was already stored.  So
the base had to be re-fetched every time after a delta CRL was added.

With this change one delta CRL to the latest base may be stored.  A newer
delta CRL will replace an existing delta CRL (but not its base, older
base CRLs are removed, though).  And a newer base will replace the existing
base and optional delta CRL.