feat(chore): singleflight when request backend multiple time at once #1270

Job	Run time
Check that Souin build as caddy module	9m 16s
Check that Souin build as middleware / Check that Souin build as Chi middleware	2m 40s
Check that Souin build as middleware / Check that Souin build as Beego middleware	2m 51s
Check that Souin build as middleware / Check that Souin build as Echo middleware	2m 55s
Check that Souin build as middleware / Check that Souin build as Dotweb middleware	2m 38s
Check that Souin build as middleware / Check that Souin build as Fiber middleware	2m 46s
Check that Souin build as middleware / Check that Souin build as Gin middleware	2m 56s
Check that Souin build as middleware / Check that Souin build as Goa middleware	2m 32s
Check that Souin build as middleware / Check that Souin build as Go-zero middleware	4m 1s
Check that Souin build as middleware / Check that Souin build as Hertz middleware	2m 26s
Check that Souin build as middleware / Check that Souin build as Kratos middleware	2m 59s
Check that Souin build as middleware / Check that Souin build as Roadrunner middleware	5m 1s
Check that Souin build as middleware / Check that Souin build as Skipper middleware	2m 39s
Check that Souin build as middleware / Check that Souin build as Souin middleware	2m 54s
Check that Souin build as middleware / Check that Souin build as Traefik middleware	2m 26s
Check that Souin build as middleware / Check that Souin build as Tyk middleware	4m 24s
Check that Souin build as middleware / Check that Souin build as Webgo middleware	2m 29s
	57m 53s

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