RFR(s): 8220774: Add HandshakeALot diag option

[Robbin Ehn]

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