Destroy kv_location before waiting for the superblock to be returned in erase_small_range. Closes #3455.
Revert "Destroy kv_location before waiting for the superblock to be returned in erase_small_range. Closes #3455."
Committed to 'next' by accident, sorry.

This reverts commit 47762c604a73f260bdec09b4b3af9ce22913ef32.
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next