Add /debug/rand endpoint

[benbjohnson]

This pull request adds a GET /debug/rand endpoint to the internal LiteFS HTTP server. When connected, this endpoint will stream bytes from a PRNG until the client disconnects or a timeout of 1 minute is encountered.

The purpose of the endpoint is to help debug slowness on a replica to see if poor network conditions are the problem.

Usage

Running curl against the endpoint will dump out performance metrics:

$ curl http://$PRIMARY:20202/debug/rand > /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 2685M    0 2685M    0     0   395M      0 --:--:--  0:00:06 --:--:--  401M