* cli: refactored snapshot list
* cli: show range tags in snapshot list
For example if N snapshots are coalesced together because they
have identical roots we may emit now:
```
2021-03-31 23:09:27 PDT ked3400debc7dd61baffab070bafd59cd (monthly-10)
2021-04-30 06:12:53 PDT kd0576d212e55a831b7ff1636f90a7233 (monthly-4..9)
+ 5 identical snapshots until 2021-09-30 23:00:19 PDT
2021-10-31 23:22:25 PDT k846bf22aa2863d27f05e820f840b14f8 (monthly-3)
2021-11-08 21:29:31 PST k5793ddcd61ef27b93c75ab74a5828176 (latest-1..3,hourly-1..13,daily-1..7,weekly-1..4,monthly-1..2,annual-1)
+ 18 identical snapshots until 2021-12-04 10:09:54 PST
```
* server: server-side coalescing of snapshot
* ui: added coalescing of retention tags
Added policy.Definition which allows us to precisely report where
each piece of policy came from.
Fixed a one-off bug with "noParent", which prevented merging of parent
policies one level too soon.
Added a whole bunch of merging helpers and generic reflection-based
test that ensures every single merge is tested.
CreateSnapshotSource API for ensuring source exists
Upload - starts upload on a given source or matching sources
Cancel - cancels upload on a given source or matching sources
