Allow to skip 'truncate table' operation using 'sequential' workload

If it is needed to skip truncation of a DB table, then just use
following new parameter:

  -skip-truncate-table

With this change it will be possible to use multiple scylla-bench
concurrent commands which won't truncate each other's data.

Closes: #30
Closes: #130

main.go

@@ -65,6 +65,7 @@ var (
 	password         string
 
 	mode           string
+	workload       string
 	latencyType    string
 	maxErrorsAtRow int
 	maxErrors      int
@@ -105,6 +106,7 @@ var (
 	hdrLatencyUnits          string
 	hdrLatencySigFig         int
 	validateData             bool
+	skipTruncateTable        bool
 )
 
 func Query(session *gocql.Session, request string) {
@@ -128,10 +130,16 @@ func PrepareDatabase(session *gocql.Session, replicationFactor int) {
 
 	}
 	if validateData {
+		if workload == "sequential" && skipTruncateTable {
+			log.Printf("Skip truncating of the '" + tableName + "' table")
+			return
+		}
 		switch mode {
 		case "write":
+			log.Printf("Truncating the '" + tableName + "' table")
 			Query(session, "TRUNCATE TABLE "+keyspaceName+"."+tableName)
 		case "counter_update":
+			log.Printf("Truncating the '" + counterTableName + "' table")
 			Query(session, "TRUNCATE TABLE "+keyspaceName+"."+counterTableName)
 		}
 	}
@@ -246,7 +254,6 @@ func getRetryPolicy() *gocql.ExponentialBackoffRetryPolicy {
 
 func main() {
 	var (
-		workload          string
 		consistencyLevel  string
 		replicationFactor int
 
@@ -315,6 +322,9 @@ func main() {
 	flag.StringVar(&hdrLatencyUnits, "hdr-latency-units", "ns", "ns (nano seconds), us (microseconds), ms (milliseconds)")
 	flag.IntVar(&hdrLatencySigFig, "hdr-latency-sig", 3, "significant figures of the hdr histogram, number from 1 to 5 (default: 3)")
 
+	flag.BoolVar(
+		&skipTruncateTable, "skip-truncate-table", false,
+		"Skip truncation of a table before running 'write' or 'counter_update' stress commands if workload is 'sequential'")
 	flag.BoolVar(&validateData, "validate-data", false, "write meaningful data and validate while reading")
 
 	var startTimestamp int64