By default the connection pool is capped only by 2 connections per host.
If a client tries to open many connections then latency increased significantly,
see https://stackoverflow.com/a/60114945/961588.
This commit configures the connection pool be unlimited but capped by 100 connections per host.
It was inspired by measurement in Caddy: https://github.com/caddyserver/caddy/issues/2805