RFR: 8278398: jwebserver: Add test to confirm maximum request time [v2]

[Julia Boes]

> This new test confirms that the jwebserver does not wait indefinitely for a request to arrive, but instead closes the connection when the maximum request time is reached. To facilitate this, Exchange::run is amended to process cancelled keys in the case where the current thread is the dispatcher thread (which is the case for the jwebserver).
> 
> Testing: tier 1-3 all clear.

Julia Boes has updated the pull request incrementally with one additional commit since the last revision:

  address PR comments:
  * remove redundant null check
  * use try-finally to stop process in test

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7052/files
  - new: https://git.openjdk.java.net/jdk/pull/7052/files/3eff7258..2b3eadd8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7052&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7052&range=00-01

  Stats: 11 lines in 2 files changed: 1 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7052.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7052/head:pull/7052

PR: https://git.openjdk.java.net/jdk/pull/7052