Add proprietary high-resolution "If-Modified-Since"

[enkore]

If this is no problem library-wise it may be better if we'd maybe use a proprietary header here? Because this isn't exactly the ETag ("nothing or everything") semantics, which aren't very useful for us here anyway. We just want last-modified without the problems of last-modified. X-Qabel-Etag-Since? Could also just stick with this.

[audax]

The proprietary header sounds better…

[enkore]

X-Qabel-Latest gives you an opaque string in GET/HEAD responses.

Specify such a string (verbatim, unmodified, treat like ETag) in the X-Qabel-New-Since field of a request and only drops newer than the latest drop of the request where that value came from is included - just like If-Modified-Since.

The resolution of this mechanism is 1 microsecond. Compared to using the database ID we don't leak the total number of drops at this time.

[enkore]

merge?