SyncChannel is the data channel in SyncJob who is responsible for constantly delivering data for one single table ( See detail in #6287 ).

At present, for each channel our sync job allocates one thread to delivery data, which is very expensive. Especially when the number of channels become large, sync job will occupy a lot of CPU resources.

Therefore, we need to use thread pool to reduce the number of threads.

But the difficulty is how to keep the sent data in the correct order. We need to a special thread pool to ensure that the tasks belonging to the same channel are executed in the order of submission.