7263: even better busy behavior (also including fix for 8575)

[jrandall]

Uses IO::Select in readfrompipes to select on a list of fds rather than slowly looping through them all.
Reaps as many children in reapchildren as are ready to be reaped, rather than one at a time.
Queries the API server for new tasks in reapchildren in a batch rather than one task at a time.

[jrandall]

Now rebased onto master for ease of merging

[tomclegg]

Github didn't follow all the rebasing and cherry-picking but all of these changes are merged in fbd5446 → 9ddb8e3 (first two commits), and ec51cd7 (third commit). Thanks @jrandall!