db_bench enhancement, when running multiple bench the creation of separate report file for each benchmark

[erez-speedb]

Owner: db_bench supports running multiple sequential benchmarks -benchmark=,, And currently there one report file only, probably for the last benchmark only.

I suggest et supporting multiple report files in the same way -report_file=,,.

[udi-speedb]

The motivation is to have a report per benchmark, and to allow easy association of the generated report with its benchmark. In addition, once https://github.com/speedb-io/speedb/issues/220 is supported, there may be multiple identical benchmarks running, each with its own configuration. Options to consider:

• Having a separate report file per benchmark (need to resolve the naming of the files)
• Having a single report file for all benchmarks, and clearly indicate within that single file where one benchmark ends and another starts