extract canvas_cassandra gem

The gem still lives in the canvas-lms repo, but has separate tests, and
knows nothing about Canvas proper. Canvas' usage of the gem is through
Canvas::Cassandra::DatabaseBuilder (and Canvas::Cassandra::Migration).

Change-Id: I51878055647225755d54edc5595b8189b7bea3cc
Signed-off-by: Stephan Hagemann <stephan.hagemann@instructure.com>
Reviewed-on: https://gerrit.instructure.com/28632
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>