Update version suffix from 'concurrency-fix' to 'ct'

Update versioning to reflect RabbitMQ.Client downgrade and adjust ver…

…sion patch number

Bump version to 8.3.6 with concurrency-fix suffix

Enhance error handling in delay message sending process. (dotnetcore#…

…1367)

Improved error handling and logging during message sending.
Wrapped `_sender.SendAsync(nextMessage)` in a try-catch block to log errors with message IDs.

Update version to 8.3.4

Update apperyor api_key

do not throw exception for existing cap-msg-group message header. ove…

…rwrite it instead. tolerate kafka message duplicate headers (dotnetcore#1623)

Fixes RabbitMQ transport reject function call error. (dotnetcore#1608)

Add more default retryable error codes for Kakfa consumption exceptio…

…ns. dotnetcore#1587

Tweak that when the `EnablePublishParallelSend` option is true, it wi…

…ll be put task into the .NET thread pool in batches rather than all at once. (dotnetcore#1540)