Replace the backtracking _PEM_BLOCK regex with a forward-only scanner
that indexes END markers by label, so repeated unterminated headers do
not cause the regex to rescan the remaining input.
Impact: 80 KiB payload went from 8.34 s to 0.001 s.
Add shared serial collision checking against existing certificates and
issuer revocations, reuse it for automatic serial allocation, and enforce
it for user-supplied serials so duplicate serials no longer break the
OCSP snapshot.
make keep_serial renewal require replace_old, refuse keep_serial +
revoke_old, and reject serials already revoked by the issuer so revoked
serials cannot be inherited by renewed certificates.
woggioni