Avoid monotime_coarse_absolute_msec in channelpadding code

As noted on legacy/trac#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.