Cleaning up HTTP/2 integration tests and examples

Motivation:

Some tests and the example code were still sending requests from the
test thread rather than from the event loop.

Modifications:

- Modified the roundtrip tests to use a new utility Http2TestUtil to
perform the write operations in the event loop thread.
- Modified the Http2Client under examples to write all requests in the
event loop thread.

Result:

Tests and examples now send requests in the correct thread context.