Use correct ActiveRecord connection for sql.active_record events

[delner]

Our previous implementation of connection configuration was always getting a default adapter configuration and using that to populate host, port and database tags for sql.active_record events.

This pull request adds functionality to retrieve more accurate information when the connection object ID is available, which is supplied in the payload for sql.active_record. With this change, we use that connection to populate the tags for ActiveRecord traces.

Such a change should represent a step forward in supporting multi-DB setups although we have yet to add support for configuring different DB connections as different services.

Example

For a request that creates the following records into different databases with sharding via active_record_shards:

The resulting trace which tags different DB connections per query. Notice the database name of highlighted span (which correlates to Article 2):

The same trace, different span (corresponding to Article 3) using other shard:

[delner]

Side note: this effectively undoes the #304 bugfix. This should pass the regression test put in place there, so I'm trusting that means the Resque issue shouldn't resurface.

[palazzem]

Great, let's test it during the week with our release candidate.

[delner]

Tested this with a sample app implementing active_record_shards to switch connections. Seems to be working fine.