Fix threading issues capturing log messages as breadcrumbs

[tustanivsky]

This PR addresses an issue which was discovered during the discussion around #514

Since log messages, asserts, ensures, etc. can come from different threads adding breadcrumbs automatically on such events can lead to undefined results due to the following reasons:

• New breadcrumb UObject instance is created during garbage collection
• Concurrent access to internal SentryScopeDesktop breadcrumbs storage

In order to resolve this breadcrumbs can now be captured without instancing any extra objects and some threading considerations were added.