ldb: move ldb_kv's filter into pack code

ldb: move ldb_kv's filter into pack code

This patch moves ldb_kv's filter code into the pack code to replace
'only attr list' functionality which will be removed in forthcoming
commit. Unpacking data then filtering the result is not any slower
than the removed 'only attr list' approach.
'only attr list' test repurposed to test unpack -> filter flow.

NOTE: Configure with --abi-check-disable to build this commit. This
patch is part of a set of LDB ABI changes, and the version update is
done on the last commit.

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