Summary:
Wrapper around `folly::netops` methods that makes it easier to mock these methods in unit tests.

When we want to mock out calls to `folly::netops` we currently:
- Use methods like `getSockOptVirtual` and `setSockOptVirtual`
- Mock part of the socket, like in the tests in `AsyncSSLSocketWriteTest`

I've the latter makes the tests particularly error prone, since we're mocking the object that's also under test.

This change introduces `netops::Dispatcher`, which is a class containing all of the functions in `folly::netops`:
- By default `AsyncSocket` uses a default, static instance of `Dispatcher` that forwards calls to the original `netops::` calls (e.g., calling `netops::Dispatcher::sendmsg` results in a call to `netops::sendmsg`.
- When a test wants to mock a a `folly::netops` call, it can call `setOverrideNetOpsDispatcher` to insert a mock `netops::Dispatcher`. I use it in this manner in D24094832

Differential Revision: D24661160

fbshipit-source-id: e9cb4ed28ffe409c74998a1c9501c0706fc853e0