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`.

core/vm/logger.go

@@ -42,6 +42,7 @@ type LogConfig struct {
 	DisableStack   bool // disable stack capture
 	DisableStorage bool // disable storage capture
 	FullStorage    bool // show full storage (slow)
+	Limit          int  // maximum length of output, but zero means unlimited
 }
 
 // StructLog is emitted to the Environment each cycle and lists information about the current internal state
@@ -64,7 +65,7 @@ type StructLog struct {
 // Note that reference types are actual VM data structures; make copies
 // if you need to retain them beyond the current call.
 type Tracer interface {
-	CaptureState(env Environment, pc uint64, op OpCode, gas, cost *big.Int, memory *Memory, stack *Stack, contract *Contract, depth int, err error)
+	CaptureState(env Environment, pc uint64, op OpCode, gas, cost *big.Int, memory *Memory, stack *Stack, contract *Contract, depth int, err error) error
 }
 
 // StructLogger is an EVM state logger and implements Tracer.
@@ -93,7 +94,12 @@ func NewStructLogger(cfg *LogConfig) *StructLogger {
 // captureState logs a new structured log message and pushes it out to the environment
 //
 // captureState also tracks SSTORE ops to track dirty values.
-func (l *StructLogger) CaptureState(env Environment, pc uint64, op OpCode, gas, cost *big.Int, memory *Memory, stack *Stack, contract *Contract, depth int, err error) {
+func (l *StructLogger) CaptureState(env Environment, pc uint64, op OpCode, gas, cost *big.Int, memory *Memory, stack *Stack, contract *Contract, depth int, err error) error {
+	// check if already accumulated the specified number of logs
+	if l.cfg.Limit != 0 && l.cfg.Limit <= len(l.logs) {
+		return TraceLimitReachedError
+	}
+
 	// initialise new changed values storage container for this contract
 	// if not present.
 	if l.changedValues[contract.Address()] == nil {
@@ -152,6 +158,7 @@ func (l *StructLogger) CaptureState(env Environment, pc uint64, op OpCode, gas,
 	log := StructLog{pc, op, new(big.Int).Set(gas), cost, mem, stck, storage, env.Depth(), err}
 
 	l.logs = append(l.logs, log)
+	return nil
 }
 
 // StructLogs returns a list of captured log entries