RFR: SSLSocket HandshakeCompletedListeners are run on virtual threads

[Carter Kozak]

Previously each `SSLSocket.addHandshakeCompletedListener(HandshakeCompletedListener)`
resulted in a new OS thread for each handshake which causes spikes in
resource utilization and bottlenecks in high throughput systems.

Virtual threads should reduce the overhead while preserving behavior to
the extent possible.

Reported to openjdk security-dev here:
https://mail.openjdk.java.net/pipermail/security-dev/2020-July/022220.html

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Commit messages:
 - SSLSocket HandshakeCompletedListeners are run on virtual threads

Changes: https://git.openjdk.java.net/loom/pull/16/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=16&range=00
  Stats: 7 lines in 1 file changed: 0 ins; 1 del; 6 mod
  Patch: https://git.openjdk.java.net/loom/pull/16.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/16/head:pull/16

PR: https://git.openjdk.java.net/loom/pull/16