Add Thread Safety and Additional Concepts README sections to Event Hubs and Service Bus

[kinelski]

The READMEs in our libraries usually contain the following sections:

### Thread safety
We guarantee that all client instance methods are thread-safe and independent of each other ([guideline](https://azure.github.io/azure-sdk/dotnet_introduction.html#dotnet-service-methods-thread-safety)). This ensures that the recommendation of reusing client instances is always safe, even across threads.

### Additional concepts
<!-- CLIENT COMMON BAR -->
[Client options](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/README.md#configuring-service-clients-using-clientoptions) |
[Accessing the response](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/README.md#accessing-http-response-details-using-responset) |
[Long-running operations](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/README.md#consuming-long-running-operations-using-operationt) |
[Handling failures](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/README.md#reporting-errors-requestfailedexception) |
[Diagnostics](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/samples/Diagnostics.md) |
[Mocking](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/README.md#mocking) |
[Client lifetime](https://devblogs.microsoft.com/azure-sdk/lifetime-management-and-thread-safety-guarantees-of-azure-sdk-net-clients/)
<!-- CLIENT COMMON BAR -->

Many of the examples and concepts in these sections are not relevant to the messaging libraries due to the references to the Azure.Core pipeline and HTTP specifics, so they haven't been added to Event Hubs and Service Bus (discussion here).

Goal: include these sections in the READMEs of the Event Hubs and Service Bus libraries with the content that's applicable to them.