[cmds][scd/store] Add 'db-manager scd-evict' subcommand enabling listing and deletion of expired SCD entities

[mickmis]

Addresses #1074

This PR adds a CLI tool db-manager scd-evict that enables deletion of expired entities. See enclosed README for more details on its usage.

Do note the approach taken when entities do not have end times: the last update times are used.

[barroco]

You will find a first round of comments. In addition, I think we could leverage the infrastructure used for RID to automate the tests you have made manually without too much effort: https://github.com/interuss/dss/blob/master/Makefile#L113.

[mickmis]

FYI still need to test manually the CLI + add unit tests.

[callumdmay]

Hey @mickmis where are we currently at with this tool? We just updated to v0.18 but we've accumulated a significant number of dangling subscriptions over the last month and they are having a significant affect on latency so we're hoping to leverage this tool ASAP

[barroco]

Hi @callumdmay, thank you for your patience. @mickmis is ooo today, he will get back to you on Monday and should be able to progress on this with top priority.

[callumdmay]

Great to hear and thanks for all the work here!

[mickmis]

Hi @callumdmay, sorry I've had to slow down significantly my work on this issue for the last few weeks. I've actually resumed that last Thursday and I expect this PR to be merged by tomorrow.

[mickmis]

@barroco this is now ready for another review. I've added unit tests and tested the tool manually in addition.

[barroco]

Looks good to me as a first version. Please take a note on the performance concern.