Improve coverage and quality of socket loop function.

[huitema]

Improve the quality of the socket loop, per issue #1603

First set of commit adds a basic test of the socket loop to the CI tests. More commits are expected to:

1. Tests for both IPv4 and IPv6
2. Unify as much as possible the Windows and Linux codes
3. Test the GSO function
4. Streamline the infrastructure support for migration and multipath
5. Test running the loop in a background thread.