Avoid unnecessary RLP encoding

While looking for the source of a memory leak, I decided to optimize
the metatracer to avoid encoding values when there weren't any tracers
configured to received the encoded values.

I also avoided putting the EVM into debug mode when there weren't any
tracers configured.

None of these fixed the memory leak, but they still seem like good
practices. Memory leak fix is in the next commit.

core/state_processor.go

@@ -71,9 +71,11 @@ func (p *StateProcessor) Process(block *types.Block, statedb *state.StateDB, cfg
 		misc.ApplyDAOHardFork(statedb)
 	}
 	blockContext := NewEVMBlockContext(header, p.bc, nil)
-	blockTracer := pluginGetBlockTracer(header.Hash(), statedb)
-	cfg.Tracer = blockTracer
-	cfg.Debug = true
+	blockTracer, ok := pluginGetBlockTracer(header.Hash(), statedb)
+	if ok {
+		cfg.Tracer = blockTracer
+		cfg.Debug = true
+	}
 	vmenv := vm.NewEVM(blockContext, vm.TxContext{}, statedb, p.config, cfg)
 	// Iterate over and process the individual transactions
 	pluginPreProcessBlock(block)