Fix interpreting scorch config: "FieldTFRCacheThreshold"

+ This threshold can be used to toggle recycling of TermFieldReaders
  on/off.
+ Couchbase users will have the ability to provide this setting within
  a JSON payload, which when interpreted into a map[string]interface{}
  will need to be interpreted as a `float64`.
+ Should library users set it as `int` we'll honor that setting as well.

index/scorch/snapshot_index.go

@@ -59,7 +59,7 @@ var reflectStaticSizeIndexSnapshot int
 // This threshold can be overwritten by users at the library level by changing the
 // exported variable, or at the index level by setting the FieldTFRCacheThreshold
 // in the kvConfig.
-var DefaultFieldTFRCacheThreshold uint64 = 10
+var DefaultFieldTFRCacheThreshold int = 10
 
 func init() {
 	var is interface{} = IndexSnapshot{}
@@ -640,10 +640,18 @@ func (is *IndexSnapshot) allocTermFieldReaderDicts(field string) (tfr *IndexSnap
 	}
 }
 
-func (is *IndexSnapshot) getFieldTFRCacheThreshold() uint64 {
+func (is *IndexSnapshot) getFieldTFRCacheThreshold() int {
 	if is.parent.config != nil {
-		if _, ok := is.parent.config["FieldTFRCacheThreshold"]; ok {
-			return is.parent.config["FieldTFRCacheThreshold"].(uint64)
+		if _, exists := is.parent.config["FieldTFRCacheThreshold"]; exists {
+			val := is.parent.config["FieldTFRCacheThreshold"]
+			if x, ok := val.(float64); ok {
+				// JSON unmarshal-ed into a map[string]interface{} will default
+				// to float64 for numbers, so we need to check for float64 first.
+				return int(x)
+			} else if x, ok := val.(int); ok {
+				// If library users provided an int in the config, we'll honor it.
+				return x
+			}
 		}
 	}
 	return DefaultFieldTFRCacheThreshold
@@ -670,7 +678,7 @@ func (is *IndexSnapshot) recycleTermFieldReader(tfr *IndexSnapshotTermFieldReade
 	if is.fieldTFRs == nil {
 		is.fieldTFRs = map[string][]*IndexSnapshotTermFieldReader{}
 	}
-	if uint64(len(is.fieldTFRs[tfr.field])) < is.getFieldTFRCacheThreshold() {
+	if len(is.fieldTFRs[tfr.field]) < is.getFieldTFRCacheThreshold() {
 		tfr.bytesRead = 0
 		is.fieldTFRs[tfr.field] = append(is.fieldTFRs[tfr.field], tfr)
 	}