RFR: 8339538: Wrong timeout computations in DnsClient

[Aleksei Efimov]

This PR proposes the following changes to address wrong timeout computations in the `com.sun.jndi.dns.DnsClient`:
- The `DnsClient` has been updated to use a monotonic high-resolution (nano) clock. The existing `Timeout` test has also been updated to use the nano clock to measure observed timeout value.

- The left timeout computation has been fixed to decrease the timeout value during each retry attempt. A new test, `TimeoutWithEmptyDatagrams`, has been added to test it.

- The `DnsClient.blockingReceive` has been updated:
    - to detect if any data is received
    - to avoid contention with `Selector.close()` that could be called by a cleaner thread

- The expected timeout calculation in the `Timeout` test has been updated to take into account the minimum retry timeout (50ms). Additionally, the max allowed difference between the observed timeout and the expected one has been increased from 50% to 67%. Taking into account 50 ms retry timeout decrease the maximum allowed difference is effectively set to 61%. This change is expected to improve the stability of the `Timeout` test which has been seen to fail [intermittentlly](https://bugs.openjdk.org/browse/JDK-8220213). If no objections, I'm planning to close [JDK-8220213](https://bugs.openjdk.org/browse/JDK-8220213) as duplicate of this one.

JNDI/DNS jtreg tests has been executed multiple times (500+) to check if the new and the modified tests are stable. No failures been observed (so far?).

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

Commit messages:
 - 8339538: Wrong timeout computations in DnsClient

Changes: https://git.openjdk.org/jdk/pull/20892/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20892&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339538
  Stats: 232 lines in 3 files changed: 214 ins; 2 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/20892.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20892/head:pull/20892

PR: https://git.openjdk.org/jdk/pull/20892