Support sharing connection pools between users

[miedzinski]

This PR adds support for sharing connection pools between users. It is done by exposing lower level pool (urllib3.PoolManager for HttpUser, geventhttpclient.client.HTTPClientPool for FastHttpUser).

Example usage:

from geventhttpclient.client import HTTPClientPool
from locust import FastHttpUser, HttpUser
from urllib3 import PoolManager


# Both users will be limited to 10 concurrent connections at most.

class UserA(HttpUser):
    pool_manager = PoolManager(maxsize=10, block=True)


class UserB(FastHttpUser):
    client_pool = HTTPClientPool(concurrency=10)

Fixes #1925.

[cyberw]

Cool stuff. A few test cases would be nice, ideally one that actually verifies somehow that connections are really pooled. And we’d need to add it to the docs somewhere.

[miedzinski]

@cyberw PTAL.

I have no idea why mypy reported that error, because I didn't touch HttpSession.request_name, but adding a type hint fixed it.

[miedzinski]

Now black is failing due to psf/black#2964.

[cyberw]

Thx! I think merging will get us the fix needed for >3.6 builds.