Allow filtering on seq range, e.g. seq=5,, seq=,10, or seq=5,10

[karmakaze]

Front-end quickvue can show duplicate rows when loading prev or next.
This can happen because we get rows <= earliest published we already have (or rows >= latest published we already have). If there are several rows with the same timestamp, the front-end only de-duplicates the adjacent row (e.g. latest fetched with earliest already had).

We can fix this by using seq for loading prev and next, then the relations will be non-overlapping (e.g. rows where seq < earliest seq we already have).

[karmakaze]

For consistency with the behaviour of published=, opted to make the ranges inclusive. This means if the client doesn't want the row matching the range limit, it must be incremented or decremented accordingly. Otherwise it could retrieve the row and merely omit it as a duplicate.