Clean up HTTP/2 integration tests

Related pull request: #2481 written by @nmittler

Motivation:

Some tests 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.
- Added HttpPrefaceHandler and its test which were missing.
- Fixed various inspector warnings

Result:

Tests and examples now send requests in the correct thread context.