[SPARK-1600] Refactor FileInputStream tests to remove Thread.sleep() calls and SystemClock usage

This patch refactors Spark Streaming's FileInputStream tests to remove uses of Thread.sleep() and SystemClock, which should hopefully resolve some longstanding flakiness in these tests (see SPARK-1600).

Key changes:

- Modify FileInputDStream to use the scheduler's Clock instead of System.currentTimeMillis(); this allows it to be tested using ManualClock.
- Fix a synchronization issue in ManualClock's `currentTime` method.
- Add a StreamingTestWaiter class which allows callers to block until a certain number of batches have finished.
- Change the FileInputStream tests so that files' modification times are manually set based off of ManualClock; this eliminates many Thread.sleep calls.
- Update these tests to use the withStreamingContext fixture.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #3801 from JoshRosen/SPARK-1600 and squashes the following commits:

e4494f4 [Josh Rosen] Address a potential race when setting file modification times
8340bd0 [Josh Rosen] Use set comparisons for output.
0b9c252 [Josh Rosen] Fix some ManualClock usage problems.
1cc689f [Josh Rosen] ConcurrentHashMap -> SynchronizedMap
db26c3a [Josh Rosen] Use standard timeout in ScalaTest `eventually` blocks.
3939432 [Josh Rosen] Rename StreamingTestWaiter to BatchCounter
0b9c3a1 [Josh Rosen] Wait for checkpoint to complete
863d71a [Josh Rosen] Remove Thread.sleep that was used to make task run slowly
b4442c3 [Josh Rosen] batchTimeToSelectedFiles should be thread-safe
15b48ee [Josh Rosen] Replace several TestWaiter methods w/ ScalaTest eventually.
fffc51c [Josh Rosen] Revert "Remove last remaining sleep() call"
dbb8247 [Josh Rosen] Remove last remaining sleep() call
566a63f [Josh Rosen] Fix log message and comment typos
da32f3f [Josh Rosen] Fix log message and comment typos
3689214 [Josh Rosen] Merge remote-tracking branch 'origin/master' into SPARK-1600
c8f06b1 [Josh Rosen] Remove Thread.sleep calls in FileInputStream CheckpointSuite test.
d4f2d87 [Josh Rosen] Refactor file input stream tests to not rely on SystemClock.
dda1403 [Josh Rosen] Add StreamingTestWaiter class.
3c3efc3 [Josh Rosen] Synchronize `currentTime` in ManualClock
a95ddc4 [Josh Rosen] Modify FileInputDStream to use Clock class.