Enforce NOT NULL on the atxs.nonce column and atomicity during sql.atxs.Add()

[ivan4th]

Description

There are a lot of tests that don't care about ATX nonces and previous ATX IDs, and because of them it's not possible to use NOT NULL on the atxs.nonce column.
Another problem is that sql.atxs.Add may cause problems if it's not used within a transaction, so it should better accept *sql.Tx instead of an Executor as an argument.
Both problems require modification of mostly same tests so using a single issue for both.

Affected code

Tests that create ATXs but don't properly set nonces and prev ATX IDs

Problem appeared after #5791