setup: extract db and http related parts in dedicated optional 'extras'

Note that for now, the main requirements.txt still contains all the dependencies for backward compatibility reasons. This will have to be fixed once all packages that depends on swh.core have been updated.

Also kill the PostgresHandler logging handler.

closes #1678 (closed).

depends on !56 (closed).

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Migrated from D1427 (view on Phabricator)