cockroachkvs: experiment with using searchDir

When `flags.TrySeekUsingNext()` is enabled, the data block iterator propagates `searchDir=+1` to `KeySeeker.Search` as a hint that if the key exists, it exists in the forward direction from the current position. Currently, we don't make use of this. We should experiment with using it to speed up seeks to successively greater keys.

Jira issue: PEBBLE-1085

Start at flags.TrySeekUsingNext(), the data block iterator, and KeySeeker.Search, where the searchDir=+1 hint is propagated. Compare successive forward seeks with and without using the hint, and measure whether the change improves seek performance without altering results.

Written by the indexing model from the issue text.