TEZ-4007: Introduce AmExtensions and Zookeeper-based FrameworkServices

[abstractdog]

This PR contains a Zookeeper-based Framework service and a major refactor in this area.

Pieces of this PR:

• include curator dependency
• remove amUuid from DAGAppMaster
• harmonized id/opaque id to externalId in terms of variable and env constant name
• FrameworkService base interface: marker interface for framework-level services, implementations are framework-specific (currently yarn or zookeeper)
  • ClientFrameworkService: interface for client-side framework services; users of this class can acquire tez clients for the actual framework (yarn/zookeeper)
  • ServerFrameworkService: interface for server-side framework services; users of this class can acquire server-side logic for the actual framework (yarn/zookeeper)
• AMExtensions: a class that encapsulates all the pluggable logic within DAGAppMaster
• update unit tests to provide an AMExtensions to prevent NPE while mocking