Add dedupe() method for a field mapping. This assures that the value is unique, even if source data contains duplicates. Commonly used for usernames. Uniqueness is currently achieved by appending a number (e.g. example1). In the future we can expand to custom replacement patterns such as email address: sally1@example.com. Dedupe can be used on any mapping.

#753284 by mikeryan: Basic implementation of sourceOfRecord==DESTINATION handling, backported from D7

Make smart timestamp handling more general, use in node migration; clear node cache to avoid out-of-memory errors