Merge: - install: recover from error on DNS server start

Close #1293

Squashed commit of the following:

commit 0981754c5c2c67f2567ee4af0d9ab24377c53413
Merge: ef81f2c8 a6d75118
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Thu Jan 16 14:19:20 2020 +0300

    Merge remote-tracking branch 'origin/master' into 1293-install

commit ef81f2c886f3bfcff4e4352d7ecea6642be7d8e1
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Dec 30 18:32:11 2019 +0300

    linter

commit 9e205be53d9de25bd2ad63398644e14b09f95238
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Dec 30 17:22:17 2019 +0300

    - install: recover from error on DNS server start

    Close all modules properly
    Don't register HTTP handlers twice

stats/stats_unit.go

@@ -114,14 +114,15 @@ func createObject(conf Config) (*statsCtx, error) {
 	}
 	s.unit = &u
 
-	s.initWeb()
-
-	go s.periodicFlush()
-
 	log.Debug("Stats: initialized")
 	return &s, nil
 }
 
+func (s *statsCtx) Start() {
+	s.initWeb()
+	go s.periodicFlush()
+}
+
 func checkInterval(days uint32) bool {
 	return days == 1 || days == 7 || days == 30 || days == 90
 }