Added logging for Flow record collections

Added a new class, FlowCollectionLogEntry, to handle logging a record collection (i.e., an instance of List<SObject>), which stores the JSON of the record collection on the LogEntry__c record. This class is very similar to FlowLogEntry and FlowRecordLogEntry, with the only difference being that it logs an instance of List<SObject>.

This release also includes a new public class, FlowLogger, that's used for sharing some code for all 3 Flow entry classes. FlowLogEntry and FlowRecordLogEntry have been refactored to leverage FlowLogger.