Don't use txid as primary key part

[elsirion]

We nearly never care about the txid, whereas the session index and item index in a session is a perfectly fine primary key for transactions too, is shorter and we even care about sesssion indices often (so it's practical if we don't have to join over transactions to find the session of a tx input/output).

fedimint-observer/schema/v0.sql

Line 35 in de6aa05

FOREIGN KEY (federation_id, txid) REFERENCES transactions(federation_id, txid), -- This might be a bit too heavy of a foreign key? Maybe use rowid instead?