Use connection pooling for metadata queries

[RPCMoritz]

When listing directories, it looks like connection pooling isn't being used.
Since I am listing a lot of directories, I am cycling through the ephemeral port list around 20 times, before doing anything else.
I would probably significantly reduce time and load on both ends, if I was able to reuse a single connection, instead of spawning 200000.

I believe the issue why connection pooling isn't being used automatically (as advertised by requests/urllib3) is that the connections created are always streaming, even if it's just a PROPFIND, which doesn't really benefit from the streaming.

My proposed solution would be to make the streaming flag in the request conditional upon the method being called. stream=True for actual up/downloads, and stream=False for list