Skip inconsistent database-driven benchmarks for now
Use scratch buffer instead of allocating in lots of places

Improvements:

benchmark                            old ns/op    new ns/op    delta
BenchmarkMockSelectString                 8883         5910  -33.47%
BenchmarkMockSelectSeries                67110        49027  -26.95%
BenchmarkMockPreparedSelectString         3467         2392  -31.01%
BenchmarkMockPreparedSelectSeries        61802        45087  -27.05%