Fix transfers to support mixins

Implement BlockchainLMDB::get_output_global_index()
- returns global output index for a given amount and amount output
  index.

Add information to debug statement for failed ring signature check
within Blockchain::check_tx_inputs()

Fixes bitmonerod RPC call "/getrandom_outs.bin" to return correct
output keys, used in creating a transaction with mixins.

TODO: get_output_global_index() could be refactored with part of
get_output_tx_and_index() as the latter uses the former's
functionality.  Keep track of LMDB read transaction.

src/cryptonote_core/blockchain.cpp

@@ -1849,7 +1849,7 @@ bool Blockchain::check_tx_inputs(const transaction& tx, uint64_t* pmax_used_bloc
     // signature spending it.
     if(!check_tx_input(in_to_key, tx_prefix_hash, tx.signatures[sig_index], pmax_used_block_height))
     {
-      LOG_PRINT_L0("Failed to check ring signature for tx " << get_transaction_hash(tx));
+      LOG_PRINT_L0("Failed to check ring signature for tx " << get_transaction_hash(tx) << "  vin key with k_image: " << in_to_key.k_image << "  sig_index: " << sig_index << "  *pmax_used_block_height: " << *pmax_used_block_height);
       return false;
     }