sampler 6.x-1.1

Changes since 6.x-1.0:

• document use of db_query_slave.
• Provide db_query_slave function if it doesn't exist, so the samplers can use it without worrying whether or not it exists.
• lock management in process method and drush sampler-sample call.
• use drush_confirm instead of custom code.
• use db_query_slave instead of db_query for all metrics computation.
• support for drush 5.x