kvserver/rangefeed: remove future package for server rangefeed

[wenyihu6]

Epic: None

[blathers-crl[bot]]

Your pull request contains more than 1000 changes. It is strongly encouraged to split big PRs into smaller chunks.

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[cockroach-teamcity]

This change is 

[blathers-crl[bot]]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{:owl: Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}