http2: export CloseIdleConnections in client connection pool

[jaricftw]

http2 transport allows stub of ClientConnPool, but both clientConnPoolIdleCloser interface and its closeIdleConnections() method are private. This makes the stubbed ClientConnPool implementation not able to implement closeIdleConnections(). As a result, it will be a no-op when
the transport call transport.CloseIdleConnections().

Therefore, exporting CloseIdleConnections() method is needed. Exporting ClientConnPoolIdleCloser interface is technically not needed, but it makes the API contract more explicit, and allows the stubbed implementation to do something like:

 _ http2.ClientConnPoolIdleCloser = (*clientConnPool)(nil)

[gopherbot]

This PR (HEAD: 3a03f30) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/net/+/233938 to see it.

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