connection state: remove temp files stranded by a crash between write and rename #825

Follow-up from #811 (#808). writeFileAtomic removes its temp file on any throw, but a process killed between creating .<record>.<uuid>.tmp and the rename leaves it in the connections directory forever. The fix removes stale temp files of that exact name shape (older than a bound, owned by no live writer) when the directory is next written or read, with a test that plants one and asserts it is removed and that a fresh one in use is not.

Refs #808, #811