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ARROW-2604: [Java] Add convenience method to VarCharVector to set Text

This adds a convenience method to easily set values in `VarCharVector` using the friendly type `Text`.  This allows the user to set values without having to think about the correct encoding.  Text objects can be constructed from a String using the constructor `Text(String string)`.

Extended existing test to set/get a `VarCharVector` using `Text` objects.

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #2071 from BryanCutler/java-varchar-from-string-ARROW-2604 and squashes the following commits:

5579edb <Bryan Cutler> add set Text to VarCharVector