move the rct commitments to the output_amounts database

Since these are needed at the same time as the output pubkeys,
this is a whole lot faster, and takes less space. Only outputs
of 0 amount store the commitment. When reading other outputs,
a fake commitment is regenerated on the fly. This avoids having
to rewrite the database to add space for fake commitments for
existing outputs.

This code relies on two things:

- LMDB must support fixed size records per key, rather than
per database (ie, all records on key 0 are the same size, all
records for non 0 keys are same size, but records from key 0
and non 0 keys do have different sizes).

- the commitment must be directly after the rest of the data
in outkey and output_data_t.

src/blockchain_db/blockchain_db.cpp

@@ -90,9 +90,8 @@ void BlockchainDB::add_transaction(const crypto::hash& blk_hash, const transacti
   // we need the index
   for (uint64_t i = 0; i < tx.vout.size(); ++i)
   {
-    amount_output_indices.push_back(add_output(tx_hash, tx.vout[i], i, tx.unlock_time));
-    if (tx.version > 1 && tx.vout[i].amount == 0)
-      add_rct_commitment(tx.rct_signatures.outPk[i].mask);
+    amount_output_indices.push_back(add_output(tx_hash, tx.vout[i], i, tx.unlock_time,
+      tx.version > 1 ? &tx.rct_signatures.outPk[i].mask : NULL));
   }
   add_tx_amount_output_indices(tx_id, amount_output_indices);
 }