Use read-only txn's instead of read lockers. Support BDB 4.4-4.7

servers/slapd/back-bdb/add.c

servers/slapd/back-bdb/back-bdb.h

servers/slapd/back-bdb/bind.c

servers/slapd/back-bdb/cache.c

servers/slapd/back-bdb/compare.c

servers/slapd/back-bdb/config.c

servers/slapd/back-bdb/delete.c

servers/slapd/back-bdb/dn2entry.c

servers/slapd/back-bdb/dn2id.c

servers/slapd/back-bdb/filterindex.c

servers/slapd/back-bdb/id2entry.c

servers/slapd/back-bdb/idl.c

servers/slapd/back-bdb/init.c

servers/slapd/back-bdb/key.c

servers/slapd/back-bdb/modify.c

servers/slapd/back-bdb/modrdn.c

servers/slapd/back-bdb/proto-bdb.h

servers/slapd/back-bdb/referral.c

servers/slapd/back-bdb/search.c

servers/slapd/back-bdb/tools.c