RFR 8204679: HTTP Client refresh

[Chris Hegarty]

Hi,

This review request consists of a number of small API changes
and implementation changes, to the new HTTP Client. This
refresh contains changes to address all recent feedback.

Summary of the significant changes:

- Make HttpHeaders final.
- More descriptive error messages have been added when a server
  misbehaves.
- Synchronous send method error handling has been updated to
  rethrow exceptions, therefore including the invoking thread's
  stack in the exception stack trace.
- Connection errors result in ConnectException
- Dependent tasks run in the common pool
- Test stabilization fixes.
- Some small performance improvements.
- General bug fixing.

https://bugs.openjdk.java.net/browse/JDK-8204679
http://cr.openjdk.java.net/~chegar/httpclient/refresh/webrev.00/

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

-Chris.

P.S. Michael is working on some last minute changes to support
TLS 1.3, that will likely be in the an updated webrev early next week.