Provide db_query_slave function if it doesn't exist, so the samplers can use it without worrying whether or not it exists.

check for existence of metric table before querying for last sample time, instead of always creating the table.

add wrapper functions for all database layer functions used in the drupal table storage handlers -- this paves the way for external databases via drupal's external database access.

add helper function from determining metric data type, use for both sampler init and ensure scheme metric function.