Fix thread-unsafe concurrent modification of m_pipes in SocketBase.ProcessTerm (#1154)

* Initial plan

* Fix thread-unsafe concurrent modification of m_pipes in SocketBase.ProcessTerm

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* Replace List<Pipe> with ImmutableArray for thread-safe m_pipes, add regression test

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* Scope System.Collections.Immutable to non-CoreApp TFMs, upgrade to 10.0.7

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* Fix two pre-existing flaky tests: MultiplePublishers ordering race and ReceiveMessageWithTimeout AutoResetEvent self-consumption

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