Backport 3338 to the 5.2 branch (#3364)

* Ensure harvesterCounter 8-byte alignment (#3273) (#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.
(cherry picked from commit fd64af2)

* Follow up with changelog for 3338 (#3363)

(cherry picked from commit 03b70c3)

CHANGELOG.asciidoc

@@ -42,6 +42,7 @@ https://github.com/elastic/beats/compare/v5.1.1...master[Check the HEAD diff]
 *Filebeat*
 - Fix registry cleanup issue when files falling under ignore_older after restart. {issue}2818[2818]
 - Fix registry migration issue from old states were files were only harvested after second restart. {pull}3322[3322]
+- Fix alignment issue were Filebeat compiled with Go 1.7.4 was crashing on 32 bits system. {issue}3273[3273]
 
 *Winlogbeat*
 - Fix for "The array bounds are invalid" error when reading large events. {issue}3076[3076]

filebeat/prospector/prospector.go

@@ -21,6 +21,8 @@ var (
 )
 
 type Prospector struct {
+	// harvesterCount MUST be first field in struct. See https://github.com/golang/go/issues/599
+	harvesterCounter uint64         // Must be 8-byte aligned. Ensured if first field in struct
 	cfg              *common.Config // Raw config
 	config           prospectorConfig
 	prospectorer     Prospectorer
@@ -30,7 +32,6 @@ type Prospector struct {
 	states           *file.States
 	wg               sync.WaitGroup
 	channelWg        sync.WaitGroup // Separate waitgroup for channels as not stopped on completion
-	harvesterCounter uint64
 }
 
 type Prospectorer interface {