Block generation period

Usage related changes

--blocks-on-demand functionality is changed to --block-generation-on <transaction|demand|<SECONDS>

now server is spawned with task::spawn and via into_future() along with create_block_interval() task when --block-generation-on <SECONDS> to handle ctrl c signal properly
mod blocks_on_demand_tests needed a rename to mod blocks_generation_tests along with filename

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[x] Performed code self-review
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[x] Updated the docs if needed - ./website/README.md
[x] Linked the issues resolvable by this PR - linking info
[x] Updated the tests if needed; all passing - execution info