mark containers as deleting before deleting them

* moved concurrent delete guard from handler to depot client
permit destruction of container while running

Signed-off-by: Alex Suraci <asuraci@pivotallabs.com>
deps

Signed-off-by: Alex Suraci <asuraci@pivotallabs.com>
Merge branch 'master' of https://github.com/cloudfoundry-incubator/executor