Add getter and setter methods for trackLastImported

I would like to add getter and setter methods for the $trackLastImported boolean in MigrateMap. This would augment the ability to set it via the constructor, as already implemented in #1703050: Enable setting trackLastImported, which is currently the only way to get at it.

I ran into the case of using migrate_d2d, which happens to handle constructing the MigrateSQLMap object without giving the calling code any way to pass $options['track_last_imported'] to the constructor.

I realize another solution could be patching migrate_d2d to add in that option, but patching Migrate to add a getter and setter seems more flexible -- unless there is some reason why allowing that setting to be changed could be problematic? The only potential issue I could see would be changing that value would lead to creating inconsistent data (i.e. some rows would have the timestamp, and others wouldn't)...but I think that could happen already by changing that option passed into the constructor between running the import multiple times, correct?