Various improvements to crash reporting

[dmick]

• some cleanups for speed and readability
• refactor database querying/data structure building into separate function
• handle multi-valued crash signatures from tracker.ceph.com
• sort by number of unique affected entities and list entities

[dmick]

Note: you can test this locally pretty easily if you have psycopg2 installed (or in a virtualenv) and a .pgpass file