FlattenJson: skip string values.

```
benchmark                                                old ns/op     new ns/op     delta
Benchmark_JsonFlattener_ContextFields-10                 17230         15985         -7.23%
Benchmark_JsonFlattener_MiddleNestedField-10             17392         16225         -6.71%
Benchmark_JsonFlattener_LastField-10                     17029         15847         -6.94%
Benchmark_JsonFlattner_Evaluate_ContextFields-10         17596         16434         -6.60%
Benchmark_JsonFlattner_Evaluate_MiddleNestedField-10     17840         16836         -5.63%

benchmark                                                old allocs     new allocs     delta
Benchmark_JsonFlattener_ContextFields-10                 23             3              -86.96%
Benchmark_JsonFlattener_MiddleNestedField-10             24             4              -83.33%
Benchmark_JsonFlattener_LastField-10                     22             2              -90.91%
Benchmark_JsonFlattner_Evaluate_ContextFields-10         33             13             -60.61%
Benchmark_JsonFlattner_Evaluate_MiddleNestedField-10     33             13             -60.61%

benchmark                                                old bytes     new bytes     delta
Benchmark_JsonFlattener_ContextFields-10                 560           48            -91.43%
Benchmark_JsonFlattener_MiddleNestedField-10             576           64            -88.89%
Benchmark_JsonFlattener_LastField-10                     544           32            -94.12%
Benchmark_JsonFlattner_Evaluate_ContextFields-10         992           480           -51.61%
Benchmark_JsonFlattner_Evaluate_MiddleNestedField-10     1000          488           -51.20%
```

flatten_json.go

@@ -189,7 +189,11 @@ func (fj *flattenJSON) readObject(pathName []byte) error {
 			var alt []byte
 			switch ch {
 			case '"':
-				val, err = fj.readStringValue()
+				if fj.skipping > 0 || !memberIsUsed {
+					err = fj.skipStringValue()
+				} else {
+					val, err = fj.readStringValue()
+				}
 				isLeaf = true
 			case 't':
 				val, err = fj.readLiteral(trueBytes)
@@ -475,6 +479,42 @@ func (fj *flattenJSON) readLiteral(literal []byte) ([]byte, error) {
 	return literal, nil
 }
 
+func (fj *flattenJSON) skipStringValue() error {
+	if fj.step() != nil {
+		return fj.error("event truncated in mid-string")
+	}
+
+	escaped := false
+	data := fj.event[fj.eventIndex:]
+
+	for i, c := range data {
+		if c == '"' {
+			if !escaped {
+				fj.eventIndex = fj.eventIndex + i
+				return nil
+			} else {
+				j := i - 1
+				for {
+					if j < 0 || data[j] != '\\' {
+						fj.eventIndex = fj.eventIndex + i
+						return nil
+					}
+					j--
+					if j < 0 || data[j] != '\\' {
+						break // odd number of backslashes
+					}
+					j--
+
+				}
+			}
+		} else if c == '\\' {
+			escaped = true
+		}
+	}
+
+	return nil
+}
+
 // we're positioned at the " that marks the start of a string value in an array or object.
 // ideally, we'd like to construct the member name as just a slice of the event buffer,
 // but will have to find a new home for it if it has JSON \-escapes
@@ -510,6 +550,8 @@ func (fj *flattenJSON) readStringValWithEscapes(nameStart int) ([]byte, error) {
 		if ch == '"' {
 			fj.eventIndex = from
 			val = append(val, '"')
+			//fmt.Printf("readStringValWithEscapes skipping [%d] value [%s]\n", fj.skipping, string(val))
+			//panic(fj.error("fuck"))
 			return val, nil
 		} else if ch == '\\' {
 			var unescaped []byte