[SPARK-6076][Block Manager] Fix a potential OOM issue when StorageLevel is MEMORY_AND_DISK_SER

In https://github.com/apache/spark/blob/dcd1e42d6b6ac08d2c0736bf61a15f515a1f222b/core/src/main/scala/org/apache/spark/storage/BlockManager.scala#L538 , when StorageLevel is `MEMORY_AND_DISK_SER`, it will copy the content from file into memory, then put it into MemoryStore.
```scala
              val copyForMemory = ByteBuffer.allocate(bytes.limit)
              copyForMemory.put(bytes)
              memoryStore.putBytes(blockId, copyForMemory, level)
              bytes.rewind()
```
However, if the file is bigger than the free memory, OOM will happen. A better approach is testing if there is enough memory. If not, copyForMemory should not be created, since this is an optional operation.

Author: zsxwing <zsxwing@gmail.com>

Closes #4827 from zsxwing/SPARK-6076 and squashes the following commits:

7d25545 [zsxwing] Add alias for tryToPut and dropFromMemory
1100a54 [zsxwing] Replace call-by-name with () => T
0cc0257 [zsxwing] Fix a potential OOM issue when StorageLevel is MEMORY_AND_DISK_SER