"Update mode" for exports

The export system in Sqoop is currently based on INSERT statements. Another option is to use Sqoop to update an existing dataset. This adds a new '--update' mode to Sqoop's export tool which generates UPDATE statements. The user specifies one column of the HDFS-resident dataset to be used as a "where" clause for UPDATE statements. Then for each record in HDFS, it executes a statement of the form "UPDATE (table) SET (all the other columns) WHERE (specified col) = (value in hdfs record)"