http: update in-memory devXX/usage on PUT+DELETE

Under heavy write traffic, free space changes constantly, and
the periodic updates every 10 (or MOG_DISK_USAGE_INTERVAL)
seconds can be too far behind.  Since we keep the usage file
contents in-memory now for out-of-FD situations, we can update
that without incurring extra VFS traffic.

v2: We no longer try to use fstatvfs(2) and instead pay the cost of
an extra name lookups and just update all usage files.  This was
necessary since calculating free space while a file is still
open can take a long time on some FSes and we need to send the
HTTP response back ASAP to avoid timeouts on the client-side.

This avoids contention in the request worker threads and the
mostly idle main thread to do more work.

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