Delivery results propagation

The goal here is to generate a file delivery.json which can be read by the workflow service to learn whatever information is pertinent about the delivery which happened at the end of a workflow. The format of the file is this (paths snipped):

{
  "delivered_to": "/.../pytest-302/test_basic_rawdata_with_tar0",
  "url": "http://localhost/dl/anonymous/ee055b22786bad3e0b8dfe5b803faf7b"
}

There are several changes here:

1. The API of destinations are modified to have a results() -> Dict method which informs the caller what happened
2. The API of Delivery is modified to return that dict
3. The main() function is modified to write that dict to delivery.json in the working directory
4. I cleaned up the output of Delivery so you get messages like the following on standard output (paths snipped again):

Delivering execution block 17A-109.sb33151331.eb33786546.57892.65940042824 to local destination /.../pytest-302/test_basic_rawdata_no_tar0 with SHA1SUM
Delivering execution block 17A-109.sb33151331.eb33786546.57892.65940042824 to tar archive in local destination /.../pytest-302/test_basic_rawdata_with_tar0 with SHA1SUM
Delivering execution block 17A-109.sb33151331.eb33786546.57892.65940042824 to shared web root at /.../pytest-302/test_web_rawdata_no_tar0/anonymous/ee055b22786bad3e0b8dfe5b803faf7b with SHA1SUM with fetch-all.sh
Delivering execution block 17A-109.sb33151331.eb33786546.57892.65940042824 to tar archive in shared web root at /.../pytest-302/test_web_rawdata_with_tar0/anonymous/ba758e2e5c7272f97937603e31578f88 with SHA1SUM with fetch-all.sh