Use consistent log field names and avoid mixing types

[bbangert]

For logging, if its the same info, it should have the same field name everywhere. Otherwise it's easy to exclude it from our parquet files accidentally. I have no strong opinion on which name is used in any of these cases, but they need a consistent name.

Here are the ones I've identified thus far:

• channel_id
• channelID
• uaid (https://github.com/mozilla-services/autopush/blob/master/autopush/web/webpush.py#L419)
• uaid_hash (https://github.com/mozilla-services/autopush/blob/master/autopush/web/webpush.py#L447)
• ttl (appears to be the same as below, but logged in a different spot)
• message_ttl

For mixed types, we use an empty string when no status or logged_status is present which can mess up analysis. We should use a known 'null' value like 0 instead of "" when logging numbers and want to log a lack of one.

[bbangert]

Closed by #883.