Test less likely condition first

Change result of MDB_IDL_ALL() to use the last entryID in the DB
instead of NOID.

DN notfound is authoritative, don't ignore the result

lookup entry id when equality on entryDN (proposed fix for ITS#5791; does not work for back-hdb, though)

as a half-way step for ITS#5037

Move db == NULL check to correct spot

Fix syncprov_findcsn to set unchecked limit. (ITS#3939)