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ARROW-662: [Format] Move Schema flatbuffers into their own file that can be included

@julienledem for some reason the Java build is failing for me locally (also on master):

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.2:testCompile (default-testCompile) on project arrow-vector: Compilation failure: Compilation failure:
[ERROR] /home/wesm/code/arrow/java/vector/src/test/java/org/apache/arrow/vector/types/pojo/TestSchema.java:[38] error: a type with the same simple name is already defined by the single-type-import of List
[ERROR] /home/wesm/code/arrow/java/vector/src/test/java/org/apache/arrow/vector/types/pojo/TestSchema.java:[64,19] error: List is abstract; cannot be instantiated
[ERROR] /home/wesm/code/arrow/java/vector/src/test/java/org/apache/arrow/vector/types/pojo/TestSchema.java:[81,19] error: List is abstract; cannot be instantiated
[ERROR] -> [Help 1]
```

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #429 from wesm/ARROW-662 and squashes the following commits:

b588f81 [Wes McKinney] Move Schema flatbuffers into their own file that can be included