Asynchronous handshakes

Some JavaThread handshake operations must be performed by only the targeted
thread(s). The most common situation is that a JavaThread should wait for
something. If the targeted JavaThread is safepoint-safe (e.g., blocked/native) it
can't start waiting and the VMThread can't wait for it.
The JavaThread may be indefinitely safepoint-safe so the handshake operation may
never be executed. The requesting thread cannot use a synchronous handshake
since it might never finish.

By letting the request be asynchronous and guarantee that the handshake
operation will be performed if the thread tries to leave the safepoint-safe state, the
requester can act as if the JavaThread was already waiting in that handshake
operation.

Right now a JavaThread can only have one handshake operation so an asynchronous
handshake would thus block all other handshakes.

This means that it's not enough to make the request asynchronous, but also a
queue is needed to be able to process concurrent handshake operations.