satellite/durability: remove fmt.Sprintf from loop observer

Small optimization.

name                 old time/op    new time/op    delta
DurabilityProcess-8    1.12ms ±12%    0.55ms ±14%  -50.99%  (p=0.008 n=5+5)

name                 old alloc/op   new alloc/op   delta
DurabilityProcess-8     300kB ± 0%     280kB ± 0%   -6.67%  (p=0.008 n=5+5)

name                 old allocs/op  new allocs/op  delta
DurabilityProcess-8     10.0k ± 0%      7.5k ± 0%  -25.00%  (p=0.008 n=5+5)

Change-Id: I8a75a315667490b255f2042fb2d781b8f4c7e75a

satellite/durability/observer.go

@@ -5,7 +5,7 @@ package durability
 
 import (
 	"context"
-	"fmt"
+	"strconv"
 	"time"
 
 	"github.com/zeebo/errs"
@@ -260,7 +260,7 @@ func (c *ObserverFork) Process(ctx context.Context, segments []rangedloop.Segmen
 
 		busFactorGroups := c.busFactorCache
 
-		streamLocation := fmt.Sprintf("%s/%d", s.StreamID, s.Position.Encode())
+		streamLocation := s.StreamID.String() + "/" + strconv.FormatUint(s.Position.Encode(), 10)
 		for classID, count := range controlledByClass {
 			if count == 0 {
 				continue