Do not acquire lock when decreasing task count.
Do not acquire lock when decreasing task count.

The tasks count decreasing (used in the limit-active-tasks strategy) can be
executed concourrently as the DB transaction will make sure that the end
value is consistent.

Trying to acquire a lock when decreasing the tasks count in a system with
several flying task can easily result in a race to acquire lock and eventually
even a "de facto" dead lock.

Signed-off-by: Alessandro Degano <55cacd5b504daf5c7cafa0b6b8e4521b86a643c4@pix4d.com>

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