ldb: <= and >= indexed searching

ldb: <= and >= indexed searching

Full implementation of <= and >= indexed searching using iterate_range
backend operation.  Adds index_format_fn to ldb_schema_syntax so
requires an ABI version bump.  The function must be provided for any
type for which <= and >= indexing is required, and must return a
lexicographically ordered canonicalization of a value.  This causes
index entries to be written in correct order to the database, so
iterate_range on the index DNs can be used.

ldb_kv_index_key is modified to return an index DN with attribute name
but without value if an empty value is provided.  This is needed for
constructing keys that match the beginning or end of an index DN range.

Pair-programmed-with: Garming Sam <garming@catalyst.net.nz>

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>