Use log instead of fmt in proxy event logger

See also:
!72 (comment 2772839)

proxy/lib/pt_event_logger.go

 package snowflake_proxy
 
 import (
-	"fmt"
 	"git.torproject.org/pluggable-transports/snowflake.git/v2/common/task"
+	"log"
 	"time"
 
 	"git.torproject.org/pluggable-transports/snowflake.git/v2/common/event"
@@ -36,7 +36,7 @@ func (p *logEventLogger) OnNewSnowflakeEvent(e event.SnowflakeEvent) {
 func (p *logEventLogger) logTick() error {
 	inbound, inboundUnit := formatTraffic(p.inboundSum)
 	outbound, outboundUnit := formatTraffic(p.inboundSum)
-	fmt.Printf("In the last %v, there are %v connections. Traffic Relayed ↑ %v %v, ↓ %v %v.\n",
+	log.Printf("In the last %v, there are %v connections. Traffic Relayed ↑ %v %v, ↓ %v %v.\n",
 		p.logPeriod.String(), p.connectionCount, inbound, inboundUnit, outbound, outboundUnit)
 	p.outboundSum = 0
 	p.inboundSum = 0