Refactor and add config for incremental model

[viplazylmht]

There are some breaking changes I make when working with dbt-vertica on incremental merge stagery:

• Refactor merge_columns to unique_key.
• Change MERGE ON logic: use HASH(cols) instead of join DEST.col1 = SRC.col1, DEST.col2 = SRC.col2,...
• Add additional config: merge_update_columns (list) to be able to update only columns user want.

[viplazylmht]

Maybe this pull request can close the issue #25

[andyreagan]

@viplazylmht this is great work. To make this easier to digest, do you think you could split this into three PRs:

1. Connection load balance changes
2. Merge updates (here, provide the HASH matching as an option).
3. Insert+delete updates (for this PR, please include why it's needed).

[andyreagan]

@viplazylmht could you push another PR here? that would trigger the automated tests.