Check for active Salesforce connection in hook_cron()

In sf_import's implementation of hook_cron(), no checking is done to see if there is an active connection with Salesforce API. Among other things, this means that the `sf_import_queue_last_import` and other sf_import timestamps are incorrectly set on each cron run, regardless of whether an actual query to Salesforce for updated items occurred.

Additionally, we might want to consider adding another variable like `sf_import_queue_last_import_attempt` to distinguish attempts from successful imports.