sf_node: Call to sf_node_import triggers call to sf_node_export

Calling sf_node_import fires node_save, which triggers the update operation in sf_node_nodeapi and results in a sf_node_export call.

An "import" should only import, not import then export.
ie. There should be a way to disable outbound nodeapi operations when performing imports.
Otherwise, we could run into recursion problems, not to mention unnecessary SalesForce API calls.

Consider a simple case where the user has enabled salesforce outbound SOAP messaging:

1. import a record from salesforce
2. save record as a drupal node
3. export the record back to salesforce
4. Salesforce triggers an outbound message back to Drupal
5. go back to #1

Consider a case of mass imports:
Using a few SalesForce API calls, I query for and gather all the SF data I want to import -- possibly thousands of records, but all returned in a relatively small number of queries. Then for each imported node, node_save() invokes a sf_node_export() -- an extra API call for each node.

Dunno if this is the optimal method, but the attached patch implements a node property "sf_node_skip_export" that short-circuits nodeapi operations on sf_node_import.