less user queries

@gwprice @e0d 

the response time of the GET user endpoint should be reduced by at least 75%, based on a load test that's not quite finished running yet.

* added some newrelic instrumentation in User.to_hash.
* some queries' results are totally ignored by the comment client so they are removed.
* an index is used to speed up the 'subscribed thread ids' query (which previously did a fair bit of disk scans).
* index on users.external_id is now unique (did not test this in isolation - contribution to overall performance improvement may be negligible).