Useful for plugins that may wish to collude with the SASL library to,
for example, retrieve a user's authorization data from a Kerberos ticket
for access control purposes

- do not send updates in state mode when
  syncCookie >= contextCSN (or omitCSN if changelog is used)

- persistent search patch
- cascading search patch

- context csn management for modrdn
- call slap_mods_opattrs() for moved entries as well

- context csn management for delete
- a later patch will take care of inconsistencies caused by
  interactions between outstanding search and scope-outs

- BDB context csn codes moved to a separate function in back-bdb/ctxcsn.c

- serialize search_candidates() with the read of search_context_csn

- currenty works for refreshOnly mode of LDAP Sync
- Context CSN for add / modify is implemented
- code for delete / modrdn / refreshAndPersist will be soon committed

(we really should be using the same filters as test003 here)

Should resolve ITS#2664.

fall into the regular send_ldap_* functionality instead of calling into
those functions explicitly. Nested callbacks still aren't handled as
cleanly as they might be.