RFR(s): 8220774: Add HandshakeALot diag option

[Robbin Ehn]

Forgot, manual tested + t1-5.

/Robbin

On 3/19/19 5:48 PM, Robbin Ehn wrote:
> Hi all, please review.
> 
> Changeset:
> http://cr.openjdk.java.net/~rehn/8220774/webrev/
> Issue:
> https://bugs.openjdk.java.net/browse/JDK-8220774
> 
> We need an option to generate a lot of handshakes for testing.
> This adds -XX:+HandshakeALot, typical usage:
> -XX:+UnlockDiagnosticVMOptions -XX:GuaranteedSafepointInterval=200 
> -XX:+HandshakeALot
> Will generate a extra handshake if there is no safepoint/handshake per 200ms.
> 
> Thanks, Robbin
>