[EventHubs] Buffered Producer concurrent sending support

[yunhaoling]

Issue

support concurrent sending in the EventHubProducerClient buffered mode

Background

there are two parameters in .NET:

• maximumConcurrentSends: The number of batches that may be sent concurrently across all partitions.
• maximumConcurrentSendsPerPartition: The number of batches that may be sent concurrently to each partition.

However, those two parameters are pulled out of the scopes of the preview release due to following reasons:

• Our current producer implementation is preventing concurrency as we lock the whole producer's send method: which makes this maximumConcurrentSendsPerPartition useless.
• The maximumConcurrentSends would also bring confusion if we cannot support concurrent sending for each partition producer. e.g. set maximumConcurrentSends to 10 but we only have 4 partition producer for each partition.
• This is an advanced perf configuration; I think only few people would be using it. We could add it incrementally.

To sum up, the concurrency story is an advanced feature which needs more time for investigation and experimenting, I prefer pulling it out of the scopes for the first preview.

Originally posted by @yunhaoling in #23748 (comment)

Scopes:

• understand the necessity of this parameter, understand whether/how customer could benefit from this parameter.
• propose a technical solution to support this feature as the current implementation of EventHubProducer could not accommodate concurrent model well (there's a big lock)
  • to enable concurrent sending, there're two options (and more) that I think we could do:
    • 1. refactor the EventHubProducer implementation to embrace concurrency with fine-grained lock
    • 2. or create multiple partition producers