RFR: 8288717: Add a means to close idle connections in HTTP/2 connection pool

[Conor Cleary]

**Issue**
When using HTTP/2 with the HttpClient, it can often be necessary to close an idle Http2 Connection before a server sends a GOAWAY frame. For example, a server or cloud based tool could close a TCP connection silently when it is idle for too long resulting in ConnectionResetException being thrown by the HttpClient.

**Proposed Solution**
A new system property, `jdk.httpclient.idleConnectionTimeout`, was added and is used to specify in Milliseconds how long an idle connection (idle connections are those which have no currently active streams) for the HttpClient before the connection is closed.

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Commit messages:
 - 8288717: Updated Property Name
 - 8288717: Add a means to close idle connections in HTTP/2 connection pool

Changes: https://git.openjdk.org/jdk/pull/10183/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10183&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288717
  Stats: 218 lines in 3 files changed: 218 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10183.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10183/head:pull/10183

PR: https://git.openjdk.org/jdk/pull/10183