RFR: 8261391: ZGC crash - SEGV in RevokeOneBias::do_thread

[Robbin Ehn]

Handshakes may contain oop Handles from the requesting thread.
A handshake may be executed by another thread than the requester.
We must make sure any such Handle is safe to use by other threads before executing the handshake.
This change-set adds SWS::start_process() for the requesting thread also (we already do this for the target JavaThread).

Passes t1-5

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Commit messages:
 - SWS start processing on handshaker

Changes: https://git.openjdk.java.net/jdk/pull/2571/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2571&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261391
  Stats: 27 lines in 1 file changed: 17 ins; 2 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2571.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2571/head:pull/2571

PR: https://git.openjdk.java.net/jdk/pull/2571