Warn on performance issues: non-normalized result set

[lgebhardt]

With the removal of the distinct call in #1225 it's now possible for a user who overrides records and joins in a has_many relationship to experience extremely slow responses due to non-normalized data. Using distinct was hiding this database access faux pas.

This PR adds detection based on the assumption that every row in the result set should translate into a unique resource fragment. If the counts do not match a warning is given in the logs. This warning can be controlled with a new configuration option, warn_on_performance_issues.

It's planned that future performance issue checks will be controlled with the same configuration setting.

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