Truncate field values to the maximum length allowed in Salesforce.

Adds a setting to control this behavior. Default is to not truncate. Setting is at admin/config/salesforce/salesforce-sync.
I do not like this section as it's very similar to what's being done in convertToSalesforceFormat. Should be refactored into a generic field property cache that can be used for checking any property.

Didn't have the heart to refactor though.