TRUNK-4109: Get observations by accession number

org.openmrs.api.ObsService#getObservations and
org.openmrs.api.ObsService#getObservationCount are now overloaded to support
specification of accession number.
If specified, only observations (or their count) with matching accession
number are returned.

See https://issues.openmrs.org/browse/TRUNK-4109

Unit tests for org.openmrs.api.ObsService#getObservations and
org.openmrs.api.ObsService#getObservationCount now use the new
(and most general) methods.