Add support for delivery rate estimation.

6 jobs for develop in 1 minute and 8 seconds (queued for 2 seconds)
Name Stage Failure
failed
test:prrt_mem Test There is an unknown failure, please try again
==3150==     in use at exit: 0 bytes in 0 blocks
==3150== total heap usage: 2,779 allocs, 2,779 frees, 954,904 bytes allocated
==3150==
==3150== All heap blocks were freed -- no leaks are possible
==3150==
==3150== For counts of detected and suppressed errors, rerun with: -v
==3150== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
sender failed
ERROR: Job failed: exit status 1