SERVER-13739: don't delete new files until as late as possible during repair
SERVER-13766 SubplanRunner::kill() needs to clear _collection

For all child classes of Runner, implementations of Runner::kill()
must guarantee the postcondition that subsequent calls to
Runner::collection() will return NULL.  SubplanRunner:kill() was not
meeting this contract.

(cherry picked from commit 1d98478d9d529e886143415cbb5b507362ab45eb)
Merge remote-tracking branch 'upstream/v2.6' into v2.6