Support cohort definitions using a strategy pattern

Background

Cohort Definition

Used to determine the people in the cohort. Determines whether the list of people is calculated on demand, calculated periodically, calculated once, or manually managed. Clients should be able to work with cohorts without having to know about or interact with definitions – i.e., just fetch a list of people in a cohort or add/remove cohort members for cohorts that support manual changes.

Requirement
**In order to allow cohorts to be defined in ways beyond a fixed set of approaches hardcoded into OpenMRS, the API should support cohort definitions using a strategy pattern (allow novel definitions to be introduced through a CohortDefinitionHandler interface):