RFR: 8301004: httpclient: Add more debug to HttpResponseInputStream

[Daniel Fuchs]

The AsyncExecutorShutdown test has been observed failing (rarely) in our CI. The root cause seems to be an assertion error in HttpResponseInputStream. More debug traces could help with diagnosing how we reached this assertion.

-------------

Commit messages:
 - 8301004

Changes: https://git.openjdk.org/jdk/pull/12169/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12169&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301004
  Stats: 16 lines in 1 file changed: 14 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12169.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12169/head:pull/12169

PR: https://git.openjdk.org/jdk/pull/12169