RFR: 8298498: sun/net/www/http/KeepAliveCache/B8291637.java fails with "Server exception terminating: java.net.SocketException: Socket closed"

[Daniel Jeliński]

This patch attempts to address a rare test failure where read from a socket succeeds immediately after sending the response.

The existing `readRequest` method did not verify if it received the entire request. The method `readOneRequest` (copied from other tests) makes sure that we read the trailing double newline before proceeding.

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Commit messages:
 - Improve request processing

Changes: https://git.openjdk.org/jdk/pull/11647/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11647&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298498
  Stats: 20 lines in 1 file changed: 13 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/11647.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11647/head:pull/11647

PR: https://git.openjdk.org/jdk/pull/11647