atc: close the workerConn when exiting
atc: close the workerConn when exiting

atc/integration tests were failing because the db could not be dropped
due to active sessions from the new workerConn

Signed-off-by: Taylor Silva <33b075afcd0610c62dcb82fe76b4c1adc69221da@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse