RFR: 8252661: Change SafepointMechanism terminology to talk less about "blocking"

[Erik Österlund]

Hi,

The SafepointMechanism class has been used to perform safepoint 
operations, originally.
Now we also perform handshake operations, and soon also concurrent stack 
processing, using
the same hooks. Therefore, names such as 
SafepointMechanism::should_block no longer sound
right, when the real question is whether it should process a pending 
operation (be that a
safepoint, handshake or whatever else).

Naming is hard, so I don't want this discussion in my concurrent stack 
processing patch.
I have a webrev with proposed naming changes to better reflect how this 
is used:
http://cr.openjdk.java.net/~eosterlund/8252661/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8252661

Thanks,
/Erik