Fix tempo-cli list block encoding and add some helpful things

[mdisibio]

What this PR does:
The tempo-cli list block --check-dupes option was broken for compressed blocks because it did not properly set the encoding. This PR fixes that but also changes the option to --scan and adds some help things: min/max object size, and ability to exit early and print current stats with CRTL+C. These things were useful during a recent investigation.

The output looks like this:

$ ./tempo-cli -c ~/tempo.gcs.yaml list block ............ --scan
ID            :  700f39bb-219a-4bfd-a7c4-24df06cd6db7
Version       :  v1
Total Objects :  273967
Data Size     :  427 MB
Encoding      :  zstd
Level         :  0
Window        :  448220
Start         :  2021-02-17 20:34:00.405224842 +0000 UTC
End           :  2021-02-17 20:38:59.463662752 +0000 UTC
Duration      :  4m59s
Age           :  18m22s
Scanning block contents.  Press CRTL+C to quit ...
Record:  100000
^C
Scanning results:
Objects scanned :  109510
Duplicates      :  0
Smallest object :  371 B
Largest object  :  2.3 MB

Which issue(s) this PR fixes:
n/a

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[joe-elliott]

scan contents of block for duplicate trace IDs and other info (warning, can be intense)

i love this warning. let's never remove it.