fix: Close SQL Alchemy connections.

[davcamer]

Summary of Changes

This addresses #868, where extractions using the Snowflake extractor would
hand until the connection was closed. I've added close() calls to the other
SQL Alchemy based extractors since it is a general expectation of SQL Alchemy
and most of the infrastructure existed to do it already.

Tests

Is there a good way of testing these extractor classes without requiring the appropriate DB to be available?

Test coverage in this project is already significantly below the goal of 70%, so make test is failing for me. This happens regardless of these changes.

Documentation

Closing connections is an expected behavior, so I did not add specific documentation.

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[feng-tao]

thanks for the fix, lgtm

[davcamer]

There is now a 9 line run of code repeated across the 6 Extractors that wrap a SQLAlchemyExtractor with extra DB specific functionality. It's this bit at the end of init that sets up the SQLAlchemyExtractor, and now the close method as well.

        self._alchemy_extractor = SQLAlchemyExtractor()
        sql_alch_conf = Scoped.get_scoped_conf(conf, self._alchemy_extractor.get_scope()) \
            .with_fallback(ConfigFactory.from_dict({SQLAlchemyExtractor.EXTRACT_SQL: self.sql_stmt}))

        self._alchemy_extractor.init(sql_alch_conf)
        self._extract_iter: Union[None, Iterator] = None

    def close(self) -> None:
        self._alchemy_extractor.close()

This could be DRYed up with a superclass, something like SQLAlchemyWrappingExtractor maybe? But the section about config could also be a function in the sql_alchemy_extractor module, which would avoid the inheritance hierarchy. I'll add a commit to do that in a bit. I'll open a separate refactor PR to explore that.