Improve multi_asset table

[kderme]

ma_tx_out is by far the most populated table (~165M entries on epoch 319). It also references the multi_asset table.

We can improve performance here. Two ideas:

• We query the multiasset and only if it doesn't exist we insert it. Querying only requires policyId and name, which compose the unique key. Most probably the query will always succeed, since minting comes first.
• We can cache a mapping of (policyId, name) -> MultiAssetId. There are 3.2M entries of multi_asset on epoch 319, so this seems like a good bargain: 3.2M entries in memory to avoid 165M queries.

Also multi_asset doesn't have any OnDeleteCascade fields, so on a rollback the db is left with unnecessary fields.

[kderme]

Done in #1094