Support "eosc get row" that gets a single row

[abourget]

Double checks that the lower bound key is actually the key you asked (otherwise returnsthe next row) so we always need to check. It's about annoying.

[maoueh]

Would it make sense to put this in eos-go directly for broader re-use by the community?

Minimally, needs to be a helper in eosc since that is used a lot in various places, voting/recast for example is one.