[DO NOT MERGE YET] XD-1650: HDFS Partitioning POC

Extends HdfsOutboundchannelAdapter: a new optional attribute called
partition-expression-generator with an SpEL which evalutes against each
message to retrieve a partition. When using this option, the actual
handler class used is PartitionHdfsStoreMessageHandler which is also a
new class. If this expression is not used, then the regular
HdfsStoreMessageHandler class is used. This allows to have one
"hdfs-outbound-channel-adapter" endpoint with this dynamic behavior.