Use monotime_coarse for transfer times and padding times

Using absolute_msec requires a 64-bit division operation every time
we calculate it, which gets expensive on 32-bit architectures.
Instead, just use the lazy "monotime_coarse_get()" operation, and
don't convert to milliseconds until we absolutely must.

In this case, it seemed fine to use a full monotime_coarse_t rather
than a truncated "stamp" as we did to solve this problem for the
timerstamps in buf_t and packed_cell_t: There are vastly more cells
and buffer chunks than there are channels, and using 16 bytes per
channel in the worst case is not a big deal.

There are still more millisecond operations here than strictly
necessary; let's see any divisions show up in profiles.

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