auth-4.9: lmdb-safe: add prefix() cursor; use it in list/lookup/get

[Habbie]

Short description

backport of #15176

draft. It works, but we have not yet decided to merge this into 4.9

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[miodvallat]

Is the builder submodule update intentional?

[Habbie]

Is the builder submodule update intentional?

no :>

[miodvallat]

And now I think you're good to go!

[coveralls]

Pull Request Test Coverage Report for Build 13590509790

Details

• 23 of 23 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on backport-15176-to-auth-4.9.x at 63.126%

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Totals
Change from base Build 13590498558:	63.1%
Covered Lines:	41780
Relevant Lines:	59427

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💛 - Coveralls