[Java] Cleanup JDBC interfaces and eliminate one memcopy for binary/varchar fields

[asfimport]

• If we use direct setting of fields, we can avoid the extra temporary buffer and memcpy by setting bytes directly.
• We should overwrite existing vectors in consumers before returning results, to avoid the possibility of closing vectors in use (or alternatively make sure we retain the underlying buffers).
• Try to eliminate some of the state in load() by moving initialization to the constructor.

Reporter: Micah Kornfield / @emkornfield
Assignee: Ji Liu / @tianchen92

PRs and other links:

• GitHub Pull Request #5152

_{Note: This issue was originally created as ARROW-6296. Please see the migration documentation for further details.}

[asfimport]

Ji Liu / @tianchen92:
Are you planning to do this yourself?

[asfimport]

Micah Kornfield / @emkornfield:
Issue resolved by pull request 5152
#5152