MM-29573: Reuse s3 client amongst all requests (#15948)

* MM-29573: Reuse s3 client amongst all requests

An http.Client is meant as a long-term object to be reused between
consecutive requests. This reduces burden on HTTP servers by reusing TCP connections
and also makes things faster by avoiding the TCP handshake for every single request.

https://mattermost.atlassian.net/browse/MM-29573

* Trigger CI

* Fix i18n

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

go.tools.mod

@@ -4,7 +4,7 @@ go 1.14
 
 require (
 	github.com/jstemmer/go-junit-report v0.9.1 // indirect
-	github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200915114419-f4421bc07461 // indirect
+	github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20201013204121-e463fcd00ba8 // indirect
 	github.com/philhofer/fwd v1.0.0 // indirect
 	github.com/reflog/struct2interface v0.6.1 // indirect
 	github.com/tinylib/msgp v1.1.2 // indirect