Use TaskCreationOptions.RunContinuationsAsynchronously

Per https://github.com/davidfowl/AspNetCoreDiagnosticScenarios/blob/master/AsyncGuidance.md#always-create-taskcompletionsourcet-with-taskcreationoptionsruncontinuationsasynchronously, it's best to create TaskCompletionSource<T> with the option TaskCreationOptions.RunContinuationsAsynchronously in most cases. This causes any awaits waiting on the task to have the continuation queued to another thread.

While this is a slight perf hit in many cases, we only use TaskCompletionSource to cover some corners around ConnectionPool scale down. The improved safety of RunContinuationsAsynchronously is preferable.