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 master in 56 minutes and 58 seconds (queued for 3 minutes and 31 seconds)
Status Job ID Name Coverage
  Build
passed #2373
build-gnutls-mit

00:28:56

passed #2371
build-no-threads-no-slapd

00:03:24

passed #2372
build-openssl-heimdal

00:24:37