Improvements to the import flow

Following improvements can be made to the Import logic

a) Make the import extensible (i.e., remove the filter from being a static member of Import and make it an instance variable of the mapper, make the mappers or variables of interest protected. )

b) Make sure that the Import calls filterRowKey method of the filter (Useful if we want to filter the data of an organization based on the row key or using filters like PrefixFilter which filter the data in filterRowKey method rather than the filterKeyValue method). The existing test case in TestImportExport#testWithFilter works with this assumption but is so far successful because there is only one row inserted into the table.

c) Provide an option to specify the durability during the import (Specifying the Durability as SKIP_WAL would improve the performance of restore considerably.) lhofhansl suggested that this should be a parameter to the import.

d) Some minor refactoring to avoid building a comma separated string for the filter args.