db_bench: seekrandom can specify --seek_nexts to read specific keys after seek.

Summary:
Add a function as tittle.
Also use the same parameter to fillseekseq too.

Test Plan: Run seekrandom using the new parameter

Reviewers: ljin, MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: rven, igor, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D25035