Option to disable ETag for certain routes?

There's a need to disable ETag for certain routes, where it is a GET but the data may still change between requests - and it is mandatory to have no caching on it.

Currently the ETag processing is done after the user's code for the response, so there's no way to remove it _after_ express has added it.

Now if we add a `Cache-Control: no-store`, the client will surely ignore the ETag, but then the ETag is generated for nothing, and it's an extra CRC or MD5 call which can be spared.

So I think there's a need to make the ETag conditional also on the existence of a "no-store".