Since Windows cannot to a SIGHUP test to check if a process is live,
the test will fail because the lock isn't detected as stale if a process with
that ID exists. Process IDs are re-used agressively on Windows, so add 500000 makes
the test extremely unlikely (if not impossible) to fail.
Since backend.ID is always a slice of constant length, use an array
instead of a slice. Mostly, arrays behave as slices, except that an
array cannot be nil, so use `*backend.ID` insteaf of `backend.ID` in
places where the absence of an ID is possible (e.g. for the Subtree of a
Node, which may not present when the node is a file node).
This change allows to directly use backend.ID as the the key for a map,
so that arbitrary data structures (e.g. a Set implemented as a
map[backend.ID]struct{}) can easily be formed.
