Adding support for Unfiltered Monitoring

Adding support for Unfiltered Monitoring

RFC 7047 supports unfiltered monitoring, wherein an application can send a Table Monitor request without specifying any
Columns to filter upon. OVSDB server will respond with TableUpdate for the requested table with data from all of its
columns (except _uuid).
In order to add this support, changes were made to MonitorRequest with an option of keeping its "columns" member as null.
Jackson serialization will ignore any member with a null value which will aid in the unfiltered monitor request.

Tests were added to cover this case as well.
Also thanks to this IT test, an existing bug is unearthed wherein the special column name was "_version" instead of "version".

Change-Id: I8e43624b09a2bf38a8846bd33ed1b493c14695ee
Signed-off-by: Madhu Venugopal <mavenugo@gmail.com>