[SPARK-5735] Replace uses of EasyMock with Mockito

This patch replaces all uses of EasyMock with Mockito.  There are two motivations for this:

1. We should use a single mocking framework in our tests in order to keep things consistent.
2. EasyMock may be responsible for non-deterministic unit test failures due to its Objensis dependency (see SPARK-5626 for more details).

Most of these changes are fairly mechanical translations of Mockito code to EasyMock, although I made a small change that strengthens the assertions in one test in KinesisReceiverSuite.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #4578 from JoshRosen/SPARK-5735-remove-easymock and squashes the following commits:

0ab192b [Josh Rosen] Import sorting plus two minor changes to more closely match old semantics.
977565b [Josh Rosen] Remove EasyMock from build.
fae1d8f [Josh Rosen] Remove EasyMock usage in KinesisReceiverSuite.
7cca486 [Josh Rosen] Remove EasyMock usage in MesosSchedulerBackendSuite
fc5e94d [Josh Rosen] Remove EasyMock in CacheManagerSuite