First round of unit tests in http2x.

Correcting minor typos in comments.

Fixing a couple of race conditions and adjusting tests timing to account for slowdown in race detection tests.

go fmt

Adding first top-level client test case.

Removing unneeded TODO.

Fixing defect in authentication verification when using client push.

Adding dial test cases.

Strict choice of PKCS format based on PEM block type (thanks to @jameshfisher)

Explicitly handling NoCallback case when closing a client.

Fixing performance polling logic for cases where MinSustain is set to 0.

Fixing minor typo in a comment.

Switching to PKCS#8 parser in crypto as it also handles keys in #1 format.

Making back-off configurable.

Adding the ability to cap exponential back-off.