add package_pending_since field to app model

 - package_pending_since gets updated everytime a package is marked as pending
 - package_pending_since is cleared (set to NULL) when package state
 chagnes to anything that isn't PENDING

[#80259334]

Signed-off-by: Johannes Petzold <jpetzold@pivotal.io>
periodically clean up packages that have been pending for too long

[#80259334]

Signed-off-by: Johannes Petzold <jpetzold@pivotal.io>
staging complete handlers fail if staging was already marked as failed

- for DEA and Diego
- staging could be marked as failed due to having been cleaned up by the pending package cleanup job

[#80259334]

Signed-off-by: Luan Santos <lsantos@pivotal.io>