Review archive schema, strengthen constraints

[georgeee]

Schema doesn't strictly reflect Mina's specifics

For example:

"blocks_user_commands_pkey" PRIMARY KEY, btree (block_id, user_command_id, sequence_no)

This primary key allows same user command to be associated with the same block a few times (whereas in practice it's at most once).

There are a few more constraints like that which could be strengthened