(ITS#7903) back-mdb should save sorted status of attrs

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: 2.4
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (78.155.233.73)
Submitted by: hyc


Currently sorted attributes get stored in sorted order but the attribute flag
indicating this is not stored. Thus an unnecessary sort also gets performed in
mdb_entry_decode when reading back from the DB. mdb_entry_encode/decode should
be changed to record this attribute flag.