introduce transfer substitution in form of custom json for cases where colony worker account has nothing on balances

Also try to look for HIVE in balance before falling back to substitution.
Note that the change only needs to reduce likelyhood of transfer being invalid, not to make sure the balance is nonzero.
We can't do that anyway (since balance can change after transaction was sent but before it was executed), so we also
don't need to care about read-locking access to balances. The whole problem with zero balance should never occur on properly
prepared testnet, however it is very frequent on mirrornet.

Author: ABW

libraries/plugins/colony/colony_plugin.cpp

@@ -84,6 +84,7 @@ struct transaction_builder
   std::array< operation_stats, NUMBER_OF_OPERATIONS > _stats;
   uint32_t                                            _reply_substitutions = 0;
   uint32_t                                            _vote_substitutions = 0;
+  uint32_t                                            _transfer_substitutions = 0;
   uint32_t                                            _failed_transactions = 0;
   uint32_t                                            _failed_rc = 0;
   uint32_t                                            _tx_num = 0;
@@ -189,6 +190,11 @@ void transaction_builder::print_stats() const
     ilog( "${r} replies and ${v} votes substituted with articles due to lack of proper target comment",
       ( "r", _reply_substitutions )( "v", _vote_substitutions ) );
   }
+  if( _transfer_substitutions )
+  {
+    ilog( "${r} transfers substituted with custom jsons due to lack of funds",
+      ( "r", _transfer_substitutions ) );
+  }
   if( _failed_transactions )
   {
     ilog( "${f} transactions failed with exception (including ${r} due to lack of RC)",
@@ -496,18 +502,33 @@ void transaction_builder::build_vote( const account_object& actor, uint64_t nonc
 
 void transaction_builder::build_transfer( const account_object& actor, uint64_t nonce )
 {
+  bool use_hive = false;
+  if( actor.get_hbd_balance().amount.value == 0 )
+  {
+    if( actor.get_balance().amount.value == 0 )
+    {
+      ++_transfer_substitutions;
+      build_custom( actor, nonce );
+      return;
+    }
+    use_hive = true;
+  }
+
   ++_stats[ TRANSFER ].count;
 
   transfer_operation transfer;
   transfer.from = actor.get_name();
   transfer.to = (*_current_account)->get_name();
-  transfer.amount = HBD_asset( 1 );
+  if( use_hive )
+    transfer.amount = HIVE_asset( 1 );
+  else
+    transfer.amount = HBD_asset( 1 );
   transfer.memo = std::to_string( nonce );
   auto extra_size = _common._params[ TRANSFER ].randomize();
   _stats[ TRANSFER ].extra_size += extra_size;
   if( extra_size > (int64_t)transfer.memo.size() + 1 )
   {
-    extra_size -= ( int64_t ) transfer.memo.size() + 1;
+    extra_size -= (int64_t)transfer.memo.size() + 1;
     std::string fill;
     fill_string( fill, extra_size );
     transfer.memo = transfer.memo + ":" + fill;