vm, ethapi: add limit option to traceTransaction

that specifies the maximum number of elements in the `structLogs`
output.  This option is useful for debugging a transaction that
involves a large number of repetition.

For example,
```
debug.traceTransaction(tx, {disableStorage: true, limit: 2})
```
shows at most the first two steps in the `structLogs`.

internal/ethapi/tracer.go

@@ -278,7 +278,7 @@ func wrapError(context string, err error) error {
 }
 
 // CaptureState implements the Tracer interface to trace a single step of VM execution
-func (jst *JavascriptTracer) CaptureState(env vm.Environment, pc uint64, op vm.OpCode, gas, cost *big.Int, memory *vm.Memory, stack *vm.Stack, contract *vm.Contract, depth int, err error) {
+func (jst *JavascriptTracer) CaptureState(env vm.Environment, pc uint64, op vm.OpCode, gas, cost *big.Int, memory *vm.Memory, stack *vm.Stack, contract *vm.Contract, depth int, err error) error {
 	if jst.err == nil {
 		jst.memory.memory = memory
 		jst.stack.stack = stack
@@ -301,6 +301,7 @@ func (jst *JavascriptTracer) CaptureState(env vm.Environment, pc uint64, op vm.O
 			jst.err = wrapError("step", err)
 		}
 	}
+	return nil
 }
 
 // GetResult calls the Javascript 'result' function and returns its value, or any accumulated error