Cache the connection we use to clean the DB with
Cache the connection we use to clean the DB with

Due to pools etc eveything we called connection_klass.connection we
would get a different connection. I hit a bug using multiple DBs where
eventually the connection that did the referential integrity would not
exist anymore and then everything would fail when it tried to reset the
foreign keys.

gentoo/database_cleaner,simonmorley/database_cleaner,avalonmediasystem/database_cleaner,yangc5/playlister-sinatra-v-000,reinaris/database_cleaner,claptimes5/database_cleaner,glebtv/database_cleaner,yangc5/playlister-sinatra-v-000,DatabaseCleaner/database_cleaner,ericraio/database_cleaner,yangc5/playlister-sinatra-v-000,yangc5/playlister-sinatra-v-000,yangc5/playlister-sinatra-v-000,HaiTo/database_cleaner,stanislaw/database_cleaner,fajrif/database_cleaner,0xCCD/database_cleaner,custora/database_cleaner,awilkening/database_cleaner,yangc5/playlister-sinatra-v-000,amorphid/database_cleaner,DatabaseCleaner/database_cleaner,eliotsykes/database_cleaner,yangc5/playlister-sinatra-v-000