* Introduce `PartitionedChannel`
* Implement a `PartitionedChannel` as an extension of the `AbstractExecutorChannel`
* Supply this channel with a `PartitionedDispatcher` which is an extension of the `AbstractDispatcher`
* The target partition is essentially a `UnicastingDispatcher` with a single thead executor
* * Fix language in Javadocs
* Add docs for `PartitionedChannel` into `channel.adoc`
* Pre-populate partitions in the `PartitionedDispatcher`
to ensure a thread number reflection of the partition it is used for (for default `ThreadFactory`)
* Add Javadocs to `public` methods
* Add Java DSL `PartitionedChannelSpec` and respective factory methods into `Channels`
* Use `IntegrationMessageHeaderAccessor.CORRELATION_ID` header as a default partition key
* * Fix language in Javadocs
* Fix Checkstyle violations
* Mark `PartitionedDispatcher.prePopulatedPartitions()` with `synchronized`
* * Populate partitions from `PartitionedDispatcher` ctor
and when a custom `ThreadFactory` is set
* * Clear `executors` in `populatedPartitions()`
* * Bring back `synchronized populatedPartitions()` and use it in the `dispatch()`
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