Rewrite the HTTP connection pool used by HttpURLConnection.

Rewrite the HTTP connection pool used by HttpURLConnection.

This started off as incremental changes, but it ended up going
far enough that it earned the "rewrite" badge.

System.getProperty() is not called for every HTTP connection.
This is slightly controversial, but the old code had to bend
over backwards to support dynamic pool changes, and it didn't
even support the case when the pool shrank but not to 0.

The new code doesn't do I/O within static synchronized blocks.
This should reduce contention described here: http://b/issue?id=2606547

Also: simpify implementation, prettier names, more focused doc.