Split IOContext.DEFAULT into WRITE and READ.

[jpountz]

This effectively removes the DEFAULT IOContext and forces callers to use the existing IOContext.READ context or the new IOContext.WRITE context instead.

This comes with some interesting changes:

• MockDirectoryWrapper fails when using a writing context for reading or a reading context for writing.
• Directory.copyFrom no longer takes an IOContext.
• LuceneTestCase#newIOContext has been split into newReadIOContext and newWriteIOContext.

[jpountz]

For what it's worth, the change is huge because lots of tests used to use IOContext.DEFAULT and had to be fixed. If you filter the diff to only look at the non-test source folders, this change is pretty contained.

[uschindler]

Hi, I was thinking about it. Why do we need to differentiate between read and write? If you open an IndexInput you read, if you open IndexOutput you're going to write. If you copy, you will do both and this sequential.

How about removing READ instead and name it DEFAULT? The others are FLUSH and MERGE.

In the future we may also have a WriteAdvice enum so we have all coordinates for handling IO.

[uschindler]

Actually my suggestion is now a rather simple rename so should be easy to apply with IDE. The diff should also get shorter.

[jpountz]

That should work as well, let me give it a try.

[jpountz]

@uschindler See #13242.

[uschindler]

Perfect. Let's close this PR as won't fix.

[jpountz]

Closed in favor of #13242.