REST: extend performance improvements to other parts of the API

REST: extend performance improvements to other parts of the API

We can trivially extend what we've just done to other parts of the API.

I haven't done much by way of benchmark but we're seeing multiple 'x's
pretty much across the board when filtering.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>
(backported from commit 046aa155c3bf827691bab9e1df8916c969a30d54
 - dropped tests, it depends on a test we don't carry
 - rejigged to suit old M:N series model and old API)
Signed-off-by: Daniel Axtens <dja@axtens.net>