db_bench: Allow running multiple benchmarks each with its own configuration

[udi-speedb]

This is a new db_bench feature
db_bench allows running multiple benchmarks in a single invocation. However, all of the benchmarks share the same configuration
This feature will allow a user to override the common configuration and have each benchmark run with its own (possibly unique) configuration.
For example (Just to demo the idea):
Common options:
option1 = A
option2 = B

Benchmarks:
b1 option1=A, option2=B
b2 option1=A1
b3 option3=C

The outcome:
b1 option1=A, option2=B, option3=default
b2 option1=A1, option2=B, option3=default
b3 option1=A, option2=B, option3=C