Update apperyor api_key

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

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

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

Add more default retryable error codes for Kakfa consumption exceptio…

…ns. #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. (#1540)

Fixed an issue where the PublishDelay synchronization method did not …

…wait internally.

Update docs.

Merge branch 'master' of https://github.com/dotnetcore/CAP

Upgrade Microsoft.Data.SqlClient to version 5.1.4

Update docs & code cleanup.