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ARROW-5917: [Java] Redesign the dictionary encoder

The current dictionary encoder implementation (org.apache.arrow.vector.dictionary.DictionaryEncoder) has heavy performance overhead, which prevents it from being useful in practice:

* There are repeated conversions between Java objects and bytes (e.g. vector.getObject).
* Unnecessary memory copy (the vector data must be copied to the hash table).
* The hash table cannot be reused for encoding multiple vectors (other data structure & results cannot be reused either).
* The output vector should not be created/managed by the encoder (just like in the out-of-place sorter)
* The hash table requires that the hashCode & equals methods be implemented appropriately, but this is not guaranteed.
We plan to implement a new one in the algorithm module, and gradually deprecate the current one.

Closes #4994 from liyafan82/fly_0712_encode and squashes the following commits:

8b699a8f7 <liyafan82>  Redesign the dictionary encoder

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>