UNDERTOW-1846 Configurable SSLEngine delegated task executor

[carterkozak]

Jira: https://issues.redhat.com/browse/UNDERTOW-1846

This provides a relatively simple mechanism to control concurrency
of the delegated task portion of a handshake. The work done by
these tasks doesn't necessarily match the type of work that the
worker pool is configured for and may be controlled separately.

The connection is immediately closed when a delegated task executor
rejects a task.

[fl4via]

@carterkozak you have my okay, I liked the approach you're proposing

[fl4via]

I'm adding the label "waiting PR update" to indicate that it is not ready to be merged yet.

[carterkozak]

Thanks, @fl4via, I ended up writing tests on this branch already, it's the PR description that describes lack of tests that's out of date. Let me know if there's any additional specific test coverage you'd like added.

I tested this out by running a server process with a profiler, and handshaking in a hot loop via new sslsockets from another process (on several threads). I verified that the majority of cpu cycles were spent within delegated tasks. Rejecting delegated tasks reduces system load and avoids blocking awaiting entropy (a bit contrived, rejecting work results in lower utilization than not rejecting work!).