Adds async versions of ReadModelBase.Start()

[joshkempner]

What does this pull request do?
Enables a mechanism to determine when a read model is live using async-await instead of using blockUntilLive in calls to Start.

How does this pull request accomplish that goal?
Adds a StartAsync method for each overload of Start in ReadModelBase. These methods perform event playback on a task pool thread. The class exposes an awaitable IsLive property that completes when all async playback Tasks have completed.

Why is this pull request important?
The use of blockUntilLive can cause problems when running unit tests in the lightweight VMs that are often used for CI builds. The problem is that Start<T>(blockUntilLive: true) synchronously blocks a taskpool thread while waiting for the stream reader to catch up. On a runner with a small number of cores, even sequential tests within one assembly can starve the pool if multiple ReadModelBase instances are constructed in quick succession since each test class creates fresh RMs in its constructor.

Link to any issues that this resolves
This does not address any open issues.

Checklist

• All unit tests in the solution must pass on all versions of .NET that the solution supports
• Any new code must be covered by at least one unit test
• All public methods must be documented with XML comments