(ITS#4834) DN matching rules should be indexed

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: 2.3, HEAD
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (76.168.84.21)
Submitted by: hyc


The dnSubtreeMatch, dnOneLevelMatch, dnSubordinateMatch, and dnSuperiorMatch
matching rules added in OpenLDAP 2.3 can be used in extended filter clauses to
get finer control over the scope of a search. Currently back-bdb/back-hdb don't
do any indexing for extended filters, which can make these filters pretty
expensive to evaluate.

But these specific rules can take advantage of the dn2id index, which already
maintains all the necessary index information. If these extensions are going to
be used frequently, we should add index support here.