RFR: 8297030: Reduce Default Keep-Alive Timeout Value for httpclient

[Conor Cleary]

Proposed changes to reduce the default Keep Alive Timeout value in `jdk/internal/net/http/HttpClientImpl.java` from 1200 seconds to 30 seconds. The current default value of 1200s is needlessly high and in remote connections that allow a client to suggest a value, this will mean that idle connections will wait very long before being dropped.

This value of 30 seconds was chosen by reviewing the current default values for Keep-Alive timeouts in common server implementations and choosing a value that would have the client's default value be lower whenever the value can be set.

Further discussion on this change can be seen the pull request [8288717: Add a means to close idle connections in HTTP/2 connection pool #10183](https://github.com/openjdk/jdk/pull/10183).

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Commit messages:
 - 8297030: Reduce Default Keep-Alive Timeout Value for httpclient

Changes: https://git.openjdk.org/jdk/pull/11285/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11285&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297030
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11285.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11285/head:pull/11285

PR: https://git.openjdk.org/jdk/pull/11285