[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
SPARK-4567. Make SparkJobInfo and SparkStageInfo serializable

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3426 from sryza/sandy-spark-4567 and squashes the following commits:

cb4b8d2 [Sandy Ryza] SPARK-4567. Make SparkJobInfo and SparkStageInfo serializable
SPARK-4805 [CORE] BlockTransferMessage.toByteArray() trips assertion

Allocate enough room for type byte as well as message, to avoid tripping assertion about capacity of the buffer

Author: Sean Owen <sowen@cloudera.com>

Closes #3650 from srowen/SPARK-4805 and squashes the following commits:

9e1d502 [Sean Owen] Allocate enough room for type byte as well as message, to avoid tripping assertion about capacity of the buffer
[SPARK-4740] Create multiple concurrent connections between two peer nodes in Netty.

It's been reported that when the number of disks is large and the number of nodes is small, Netty network throughput is low compared with NIO. We suspect the problem is that only a small number of disks are utilized to serve shuffle files at any given point, due to connection reuse. This patch adds a new config parameter to specify the number of concurrent connections between two peer nodes, default to 2.

Author: Reynold Xin <rxin@databricks.com>

Closes #3625 from rxin/SPARK-4740 and squashes the following commits:

ad4241a [Reynold Xin] Updated javadoc.
f33c72b [Reynold Xin] Code review feedback.
0fefabb [Reynold Xin] Use double check in synchronization.
41dfcb2 [Reynold Xin] Added test case.
9076b4a [Reynold Xin] Fixed two NPEs.
3e1306c [Reynold Xin] Minor style fix.
4f21673 [Reynold Xin] [SPARK-4740] Create multiple concurrent connections between two peer nodes in Netty.
Config updates for the new shuffle transport.

Author: Reynold Xin <rxin@databricks.com>

Closes #3657 from rxin/conf-update and squashes the following commits:

7370eab [Reynold Xin] Config updates for the new shuffle transport.