[Java] Implement a <Object, int> HashMap for DictionaryEncoder

[asfimport]

As a follow-up of ARROW-5726. Implement a Map<Object, int> for DictionaryEncoder to reduce boxing/unboxing operations.

Benchmark:
DictionaryEncodeHashMapBenchmarks.testHashMap: avgt 5 31151.345 ± 1661.878 ns/op
DictionaryEncodeHashMapBenchmarks.testDictionaryEncodeHashMap: avgt 5 15549.902 ± 771.647 ns/op

Reporter: Ji Liu / @tianchen92
Assignee: Ji Liu / @tianchen92

Related issues:

• [Java] Apply new hash map in DictionaryEncoder (is a parent of)

PRs and other links:

• GitHub Pull Request #4765

_{Note: This issue was originally created as ARROW-5814. Please see the migration documentation for further details.}

[asfimport]

Micah Kornfield / @emkornfield:
Issue resolved by pull request 4765
#4765