lib ldb: rename ltdb_private to ldb_kv_private

lib ldb: rename ltdb_private to ldb_kv_private

Rename ltdb_private to ldb_kv_private as it contains key value operation
context.

Note there is still some tdb specific context that can be refactored into a
separate structure along the lines of the lmdb context.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>