Fixing startup race condition in Http2ConnectionRoundtropTest

Motivation:

The HTTP/2 tests have been unstable, in particular the
Http2ConnectionRoundtripTest.

Modifications:

Modified the bootstrap code for the client and server so that we don't
swap out the CountDownLatches for request/data.  I believe this was
leading to race conditions where the test was using different latches
than the server.

Result:

Tests should (hopefully) be more stable.