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ARROW-1237: [JAVA] expose the ability to set lastSet

The changes here expose the ability to set "lastSet" on Nullable<var length>Vector.
I believe this is needed only for NullableVarCharVector and NullableVarBinaryVector. Hence the API is exposed through NullableValueVectors.java

Author: siddharth <siddharth@dremio.com>

Closes #868 from siddharthteotia/ARROW-1237 and squashes the following commits:

786dfea [siddharth] ARROW-1237: addressed review comments and added more tests
73b2fc5 [siddharth] ARROW-1237: added some unit tests
f8c7277 [siddharth] ARROW-1237: expose the ability to set lastSet