Fix `MonotonicTimestampGenerator` test and remove `src/timestamp_generator.[cpp/hpp]`

[muzarski]

This test implements its custom wrapper over MonotonicTimestampGenerator (defined in src/timestamp_generator.cpp). The wrapper's next() implementation simply appends generated timestamp to vector. This vector is then inspected during the test.

To remove the files mentioned in PR title, we firstly need to implement some testing utility API on rust side which would allow to collect the generated timestamps and inspect them. The test needs to be rewritten using this API.

All this should be done once #295 is merged.

[Lorak-mmk]

Do we have any integration tests (= tests that connect to DB) written in Rust? Or are all in C++, and only unit tests are in Rust?
I'm asking because maybe it would be easier in such cases to rewrite such test in Rust.

[muzarski]

Do we have any integration tests (= tests that connect to DB) written in Rust? Or are all in C++, and only unit tests are in Rust? I'm asking because maybe it would be easier in such cases to rewrite such test in Rust.

We don't have any rust integration tests yet.