Implement TCP and UDP sockets in parent process

[sammacbeth]

The current TCPSocket and UDPSocket implements run in the extension child process. This is fine for debugging, but we found that when running in a release browser with the process sandbox enabled then this process was blocked from opening sockets on Mac and Linux, meaning the APIs did not work.

This PR fixes this issue by implementing both TCPSocket and UDPSocket APIs in the parent process. The API exposed by the child to the extension has not changed, but now everything is shipped up to the parent, which then pushes data back to the child.

• Whether this introduces an additional performance cost, I do not know. I didn't do any performance benchmarks, but also didn't see much change in the dat-webext extension after adopting these changes.
• The original test suite is passing as-is, but there may be untested regressions I have overlooked. I'm not 100% that the error handling behaves as before.
• I omitted flow types for the internal parent-process APIs. If you think they're useful I could add that. The flow check is passing at the moment.

[soyuka]

Are you stopping the work on the matter or just moving on to another repository?

Nvm libdweb#1

[sammacbeth]

@soyuka See #109 (comment) for context.