performance: implement backend statistics
We want to be able to measure the following:
- looking for reservations took X us,
- looking for leases took Y us.
- Z queries per packet were conducted.
- W total queries performed by backend, average response time was A.
- possibly stats by query type (getLease4byHWAddr, getLease4ByAddr, etc.)
- possibly query by SQL type (A number of SELECTs, B number of INSERTs, C number of DELETEs)
This, on its own, wouldn't improve any performance, but it will be an essential tool for assessing other performance improvement proposals.
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