RFR: 8288717: Add a means to close idle connections in HTTP/2 connection pool [v6]

[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.

Conor Cleary has updated the pull request incrementally with one additional commit since the last revision:

  8288717: Added NumberFormatException log, refactored ConnectionPool

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10183/files
  - new: https://git.openjdk.org/jdk/pull/10183/files/24c57157..494a9a7e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10183&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10183&range=04-05

  Stats: 27 lines in 3 files changed: 16 ins; 2 del; 9 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