Migrate EMR API merge patient visits functionality into OpenMRS Core

The EMR API module provides it's own "mergePatients" functionality that provides additional merge functionality on top of the core Patient Service mergePatients:

https://github.com/openmrs/openmrs-module-emrapi/blob/master/api/src/main/java/org/openmrs/module/emrapi/adt/AdtServiceImpl.java#L551

This provides two main pieces of functionality:

1) The ability to define custom PatientMergeAction
and
2) It merges any overlapping visits between the two patients

We'd like to migrate both of these into OpenMRS Core. TRUNK-5220 handles PatientMergeAction, this ticket is about adding the functionality to merge overlapping visits to the PatientService mergePatients method.