[SPARK-10612][SQL] Add prepare to LocalNode.

[rxin]

The idea is that we should separate the function call that does memory reservation (i.e. prepare) from the function call that consumes the input (e.g. open()), so all operators can be a chance to reserve memory before they are all consumed.

[SparkQA]

Test build #42475 has finished for PR 8761 at commit f2d4a1d.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[zsxwing]

Why not implement this one in UnaryLocalNode? Then we don't need to duplicate them in all subclasses of ``UnaryLocalNode`.

[rxin]

Actually then we could just implement it in LocalNode itself since it can traverse the children.

[rxin]

OK updated - I put everything in LocalNode.

I was initially thinking it might be better to be more explicit - but it seems vast majority of the operators don't need to do anything.

[SparkQA]

Test build #42493 has finished for PR 8761 at commit 90f7943.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[andrewor14]

retest this please

[andrewor14]

Actually this patch is essentially a no-op so I'm just going to merge it. This is going to master.

[SparkQA]

Test build #42512 has finished for PR 8761 at commit 90f7943.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.