storage prefix iter: skip keys that don't match the full prefix segments

[tzemanovic]

The prefix iter functions in the DB (both mock and rocksdb) currently also match keys that contain extra characters in the last segment of the prefix. E.g. iter prefix "a" would match "a/sub" and also "ab/sub". This is inconsistent with the prefix iterator that are not yet committed from the write log and also with the storage::Key::split_prefix function where only "a/sub" would be matched.

To make these consistent, the prefix matching logic in the DB should add a trailing slash to the prefix to avoid matching prefixes with extra characters.