Add descriptors benchmark script

Some things might need to be changed, like downloader to don't use mirrors since the extra-info descs are using endpoints.

I wanted to run it for 1h to multiply per 24 and compare with nickm numbers in the issue, but didn't wait for it cause it looks like it takes ages for some extra-info descs, maybe cause of the previous.

It might also have some other errors, since usually getting all descriptors is immediate, but here after 30min running, only got 49.

The end of the log:

2021-04-08 16:06:51,773 benchmark[19311]: <INFO> (MainThread) benchmark.py:261 - stop_threads - KB read: 0.0, KB written: 0.0, KB all: 1798.081, KB consensus: 1539.64, KB extrainfo: 116.768, KB serverdesc: 141.673, seconds elapsed: 2530.8659102916718, seconds consensus: 0.9265780448913574, seconds extrainfo: 2529.312867164612, seconds serverdescs: 0.6264650821685791.dirauth failures: 0, extrainfo failures: 0, extrainfo success: 48 , serverdescs failures: 0serverdescs success: 49, routerstatus failures: 0routerstatus success: 6723

I've assigned @mikeperry as reviewer, but feel free to change with @gk.