ldb_mdb: Apply LMDB key length restrictions at key-value layer

ldb_mdb: Apply LMDB key length restrictions at key-value layer

We need to enforce the GUID index mode so end-users do not get a
supprise in mid-operation and we enforce a max key length of 511 so
that the index key trunctation is done correctly.

Otherwise the DB will appear to work until a very long key (DN or
index) is used, after which it will be sad.

Because the previous ldb_lmdb_test confirmed the key length by
creating a large DN, those tests are re-worked to use the GUID index
mode.  In turn, new tests are written that create a special DN around
the maximum key length.

Finally a test is included that demonstrates that adding entries to
the LMDB DB without GUID index mode fails.

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