Benchmarks: Make number of records configurable via config

[QuantumDancer]

Currently, the benchmarking code provides several functions (real_case_record_size_*) that run the benchmarks with different numbers of records. If one wants to switch to a different number of records, the call to the function needs to be changed and the benchmark recompiled.

It would be nicer if we could specify the number of records (and maybe other parameters that are different in these functions) inside the config. This way, one can run different benchmarks without recompiling. Also, we then can use this to run a benchmark with a very small number of records for the CI test (see #774)