[SPARK-4765] Make GC time always shown in UI.

This commit removes the GC time for each task from the set of
optional, additional metrics, and instead always shows it for
each task.

cc pwendell

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #3622 from kayousterhout/gc_time and squashes the following commits:

15ac242 [Kay Ousterhout] Make TaskDetailsClassNames private[spark]
e71d893 [Kay Ousterhout] [SPARK-4765] Make GC time always shown in UI.
[SPARK-4714] BlockManager.dropFromMemory() should check whether block has been removed after synchronizing on BlockInfo instance.

After synchronizing on the `info` lock in the `removeBlock`/`dropOldBlocks`/`dropFromMemory` methods in BlockManager, the block that `info` represented may have already removed.

The three methods have the same logic to get the `info` lock:
```
   info = blockInfo.get(id)
   if (info != null) {
     info.synchronized {
       // do something
     }
   }
```

So, there is chance that when a thread enters the `info.synchronized` block, `info` has already been removed from the `blockInfo` map by some other thread who entered `info.synchronized` first.

The `removeBlock` and `dropOldBlocks` methods are idempotent, so it's safe for them to run on blocks that have already been removed.
But in `dropFromMemory` it may be problematic since it may drop block data which already removed into the diskstore, and this calls data store operations that are not designed to handle missing blocks.

This patch fixes this issue by adding a check to `dropFromMemory` to test whether blocks have been removed by a racing thread.

Author: hushan[胡珊] <hushan@xiaomi.com>

Closes #3574 from suyanNone/refine-block-concurrency and squashes the following commits:

edb989d [hushan[胡珊]] Refine code style and comments position
55fa4ba [hushan[胡珊]] refine code
e57e270 [hushan[胡珊]] add check info is already remove or not while having gotten info.syn