SERVER-13756 Remove DiskLoc::writing() and getDur().writingDiskLoc()
Kill ExtentManager::get(Next|Prev)Record(InExtent)? and preallocateAFile
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.