Allow the compression codec to be specified in SequenceFiles

The dfs sink writes sequence files but doesn't honour flume.collector.dfs.compress.codec. On the other hand, the customdfs sink supports compression (via flume.collector.dfs.compress.codec), but doesn't write SequenceFiles.

This can be fixed by either changing the dfs sink to honour flume.collector.dfs.compress.codec (compression would be enabled by setting io.seqfile.compression.type), or by creating a SequenceFileOutputFormat for use with the customdfs sink.