Ask for more bytes in our initial request

I don't know how we got this figure, but it seems really low:

# The number of bytes to initially request from the server
initial_read_request = 16384

Let's try to work out something more reasonable:

RELAY_REQUIRED_MIN_BANDWIDTH is 75 * 1024 = 76800 bytes per second. sbws wants measurements to take 6 seconds: 76800 * 6 = 460800. (or 76800 * 8 = 614400 after #30231 (moved).)

Downloading 1 MB on the current Tor network takes between 1-8 seconds: https://metrics.torproject.org/torperf.html?start=2019-01-18&end=2019-04-18&server=public&filesize=1mb

And 5 MB takes between 5-30 seconds: https://metrics.torproject.org/torperf.html?start=2019-01-18&end=2019-04-18&server=public&filesize=5mb

Most relays self-reported bandwidth is more than 500 kilobytes per second: https://metrics.torproject.org/advbwdist-relay.html?start=2019-01-18&end=2019-04-18&n=5000

So I suggest 1 MB:

# The number of bytes to initially request from the server
initial_read_request = 1048576