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/berkeleydb/db_bdb.cpp

@@ -361,7 +361,7 @@ void BlockchainBDB::remove_transaction_data(const crypto::hash& tx_hash, const t
         throw1(DB_ERROR("Failed to add removal of tx outputs to db transaction"));
 }
 
-void BlockchainBDB::add_output(const crypto::hash& tx_hash, const tx_out& tx_output, const uint64_t& local_index, const uint64_t unlock_time)
+void BlockchainBDB::add_output(const crypto::hash& tx_hash, const tx_out& tx_output, const uint64_t& local_index, const uint64_t unlock_time, const rct::key *commitment)
 {
     LOG_PRINT_L3("BlockchainBDB::" << __func__);
     check_open();