RFR: 8222469: ZGC: Generalize ZPageCache::flush()

[Per Liden]

Yet another off-live review by Stefan. Broke out inner/common function.

Updated webrev:
Diff: http://cr.openjdk.java.net/~pliden/8222469/webrev.1.diff
Full: http://cr.openjdk.java.net/~pliden/8222469/webrev.1

/Per

On 04/15/2019 10:15 AM, Per Liden wrote:
> In preparation for JEP 351, generalize ZPageCache::flush() so that a 
> closure can be passed in, which dictates what should/shouldn't be flushed.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8222469
> Webrev: http://cr.openjdk.java.net/~pliden/8222469/webrev.0
> 
> /Per