When `NodeLiveness` updates the liveness record (e.g. during
heartbeats), it first does a noop sync write to all disks. This ensures
that a node with a stalled disk will fail to maintain liveness and lose
its leases.

However, this sync write could block indefinitely, and would not respect
the caller's context, which could cause the caller to stall rather than
time out. This in turn could lead to stalls higher up in the stack,
in particular with lease acquisitions that do a synchronous heartbeat.

This patch does the sync write in a separate goroutine in order to
respect the caller's context. The write operation itself will not
(can not) respect the context, and may thus leak a goroutine. However,
concurrent sync writes will coalesce onto an in-flight write.

Additionally, this runs the sync writes in parallel across all disks,
since we can now trivially do so. This may be advantageous on nodes with
many stores, to avoid spurious heartbeat failures under load.

Release note (bug fix): Disk write probes during node liveness
heartbeats will no longer get stuck on stalled disks, instead returning
an error once the operation times out. Additionally, disk probes now run
in parallel on nodes with multiple stores.