Avoid monotime_coarse_absolute_msec in channelpadding code
As noted on #24374 (moved), monotime_coarse_absolute_msec() can be expensive on 32-bit systems since it has to do a 64-bit division to compute msec. We fixed the use of monotime_coarse_absolute_msec in buf_t and packed_cell_t, but we still have it in the channelpadding system.
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