Issue #7867: Move EngineSession from SessionManager to BrowserState.
Co-authored-by:
Christian Sadilek <christian.sadilek@gmail.com>
Issue #8121: Replace MigrationStore with MigrationContext and prevent usage outside of a Middleware.
* Before executing the reducer chain we now verify that we are on the store thread and throw if we are on a different thread.
* MigrationContext now provides a store property that returns the underlying Store instance. This can be used to pass it to
other components and threads that are not part of the Middleware.
* Fixed existing Middleware implementations to use the MiddlewareContext or pass the actual Store instance around.