RFR [11] 8202423: Small HTTP Client refresh

[Chris Hegarty]

A number of implementation changes have accumulated in the
sandbox since the initial integration of the HTTP Client.
The changes mainly consist of:

  - Test stabilization fixes.
  - Clean up of code left over from early prototyping.
  - Some small performance improvements.
  - General bug fixing.

https://bugs.openjdk.java.net/browse/JDK-8202423
http://cr.openjdk.java.net/~chegar/8202423/00/webrev/

I author only a small portion of these changes, and have
started, and will continue, to review the rest.

-Chris.