Combine registration of schema with registration of metadata to have tighter constraints and simplify indexer logic

[alvrs]

pulled out from #760

Can we tighten constraints around schema + metadata? i.e. always register key schema + value schema + metadata together (like a regular table), so we can reliably expect them to exist together, and not allow for them to be written more than once. -> yes

[holic]

We chatted earlier, just summarizing here:

Since this change is tightly coupled to things like store-cache, current and future networking code, etc., we decided to punt for a little while longer until the new networking code is complete (leaning on some workarounds for lack of key name, that should hopefully clear up with #1025 and an expanded config).

In the meantime, do we wanna add key names via #1059? Or still not worth the churn? Decided it was too big a change to be worth going through with.

[alvrs]

closing on in favor of #1122