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ARROW-1962: [Java] Adding reset to ValueVector interface

Adding `reset()` to the ValueVector interface and implementing where it is not done already.  Removing unused abstract class BaseDataValueVector that is not used anymore by the UnionVector.

Expanded reset tests to check that valueCount is 0, and buffers have same capacity and zeroed out.

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #1455 from BryanCutler/java-reset-ValueVector-ARROW-1962 and squashes the following commits:

da994e1 [Bryan Cutler] typo
a52e7db [Bryan Cutler] expanded reset documentations
1526a83 [Bryan Cutler] improved vector reset testing
a251d10 [Bryan Cutler] reset should zero data buffer and set value count to 0
bf2a16a [Bryan Cutler] add reset to NullableMapVector to zero validity buffer
7fbde5b [Bryan Cutler] need to zero out vector buffers when reset
b59addf [Bryan Cutler] adding reset to ValueVector interface, removing BaseDataValueVector