ttxdb: query by application metadata

[adecaro]

Currently, the ttxdb stored the metadata as a json structure. This makes more complex running select by this metadata.
The issues asks to store the metadata in a different table and then allow select by metadata key.

[arner]

Interesting idea! Do you mean select by key or also query by value? Right now we retrieve the values for a transaction, but they're marshalled into a json string. A more normalized storage would make sense, from a data perspective.

So the table would be something like: txid - idx - mdkey - mdval? We should check how to do efficient SELECTS that way, it seems that sql/database doesn't have a native way to put a one-to-many relationship into a slice. But I'm sure we can make it work.

Note that postgres supports json queries (but then, what do we do with sqlite...):

SELECT jdoc->'guid', jdoc->'name' FROM api WHERE jdoc @> '{"company": "Magnafone"}';

(source: https://www.postgresql.org/docs/current/datatype-json.html)