add Task-based DisconnectAsync and refactor APM methods on top of it

[geoffkizer]

Add Task-based DisconnectAsync and refactor old APM Begin/EndDisconnect methods on top of it.

Also, generalize and improve disconnect tests.

Note that cancellation is not currently wired up for DisconnectAsync, even though it takes a CancellationToken. I'll file a separate issue for this.

Fixes #1608
Contributes to #43845

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Issue Details

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Add Task-based DisconnectAsync and refactor old APM Begin/EndDisconnect methods on top of it.

Also, generalize and improve disconnect tests.

Note that cancellation is not currently wired up for DisconnectAsync, even though it takes a CancellationToken. I'll file a separate issue for this.

Fixes #1608
Contributes to #43845

Author:	geoffkizer
Assignees:	-
Labels:	area-System.Net.Sockets, new-api-needs-documentation
Milestone:	-

[geoffkizer]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[geoffkizer]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[geoffkizer]

@antonfirsov -- I added a new helper TaskToApmBeginWithSyncExceptions that we should be able to use to address #47905 in general (but I've only implemented it for BeginDisconnect for now).