add time measurements for all steps in bootstrapping and build-sequence

[shubhbapna]

Right now we only measure the amount of time taken to build a wheel, we should also measure time for other steps during bootstrapping and build-sequence to help us detect performance bottlenecks. Maybe we should also measure other performance counters like memory usage?

[shubhbapna]

This could be interesting to get the actual memory usage when building a wheel: https://docs.python.org/3/library/resource.html#resource-usage

[dhellmann]

It would be good to produce some aggregate info at the end of the run, too. "bootstrapping took X time, Y in building wheels, Z in building sdists, etc." for whatever we collect.

[shubhbapna]

using cgroups might be a better way because it will let us track any child processes spawned