[Java] Implement a common interface for int vectors

[asfimport]

Now in DictionaryEncoder#encode it use reflection to pull out the set method and then set values. 

Set values by reflection is not efficient and code structure is not elegant such as

Method setter = null;
for (Class<?> c : Arrays.asList(int.class, long.class)) {
try {
setter = indices.getClass().getMethod("setSafe", int.class, c);
break;
} catch (NoSuchMethodException e) {
// ignore
}
}

Implement a common interface for int vectors to directly get set method and set values seems a good choice.

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

PRs and other links:

• GitHub Pull Request #4698

_{Note: This issue was originally created as ARROW-5726. Please see the migration documentation for further details.}

[asfimport]

Micah Kornfield / @emkornfield:
Issue resolved by pull request 4698
#4698