fix MemcacheClient.FetchReports miss & leak on corrupt report

Problem: Decoding a corrupt report grows the 'missing' list. Since we
are waiting for 'len(keys)-len(missing)' decoder go-routines, this
results in waiting for fewer go-routines than we should. The surplus
go-routines leak and we ignore their reports. And since the keys of the
ignored reports are not included in 'missing', we won't attempt to fetch
them from S3 either. Oops.

Fix: calculate the number of go-routines once, at the beginning.

app/multitenant/memcache_client.go

@@ -186,7 +186,8 @@ func (c *MemcacheClient) FetchReports(keys []string) (map[string]report.Report,
 	}
 
 	reports := map[string]report.Report{}
-	for i := 0; i < len(keys)-len(missing); i++ {
+	lenFound := len(keys) - len(missing)
+	for i := 0; i < lenFound; i++ {
 		r := <-ch
 		if r.report == nil {
 			missing = append(missing, r.key)