[Java] Resolve new vector classes structure for timestamp, date and maybe interval

[asfimport]

Personally I think having 8 vector classes for timestamps is not great. This is discussed at some point during the PR:

#1203 (comment)

Reporter: Li Jin / @icexelloss
Assignee: Li Jin / @icexelloss

PRs and other links:

• GitHub Pull Request #1330

_{Note: This issue was originally created as ARROW-1816. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
I think the issue here is to avoid branching at the array cell for Java. If there is a switch branch at the hot path for accessing values, then the JIT may generate worse code

[asfimport]

Li Jin / @icexelloss:
I will look at cell level a bit closer, my hunch is branching can be avoided by storing a timeUnit object in the vector:
https://github.com/apache/arrow/blob/master/java/vector/src/main/java/org/apache/arrow/vector/NullableTimeStampNanoVector.java#L116

timeZone doesn't seem to be used in cell level either, but I need to look closer.

[asfimport]

Li Jin / @icexelloss:
Won't fix per discussion:
https://docs.google.com/document/d/1n4qjO20wZyS7wSpISgYdIVuD22zstLgP-7gXxS5_7_E/edit