Ensure harvesterCounter 8-byte alignment (elastic#3273) (elastic#3338)

harvesterCounter is accessed atomically and will fault on x86-32 or ARM
if not 8-byte aligned. See golang/go#599 for more details on why it
fails and https://golang.org/pkg/sync/atomic/#pkg-note-BUG for how
putting the field first in the struct fixes it.

filebeat/prospector/prospector.go

@@ -23,6 +23,7 @@ var (
 )
 
 type Prospector struct {
+	harvesterCounter uint64         // Must be 8-byte aligned. Ensured if first field in struct
 	cfg              *common.Config // Raw config
 	config           prospectorConfig
 	prospectorer     Prospectorer
@@ -32,7 +33,6 @@ type Prospector struct {
 	states           *file.States
 	wg               sync.WaitGroup
 	channelWg        sync.WaitGroup // Separate waitgroup for channels as not stopped on completion
-	harvesterCounter uint64
 	ID               uint64
 	Once             bool
 }