ITS#9342 delta-sync: ignore add if entryCSN is too old

This check is only needed for ops received without a CSN in their cookie.
This only occurs when the ops completed out of order on the provider.
3 jobs for its9342b in 33 minutes and 47 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #2365
build-gnutls-mit

00:30:09

passed #2363
build-no-threads-no-slapd

00:03:37

passed #2364
build-openssl-heimdal

00:30:59