Fix 'slice bounds out of range' in DeleteMessageBatch

[forrest-ua]

There is a bug inside DeleteMessageBatch function (issue #171):

2018/09/21 14:58:01 http: panic serving 172.18.0.1:42200: runtime error: slice bounds out of range
goroutine 22 [running]:
net/http.(*conn).serve.func1(0xc00015e0a0)
	/usr/local/go/src/net/http/server.go:1746 +0xd0
panic(0x70d980, 0x9cf310)
	/usr/local/go/src/runtime/panic.go:513 +0x1b9
github.com/p4tin/goaws/app/gosqs.DeleteMessageBatch(0x7c75e0, 0xc000138620, 0xc000137c00)
	/go/src/github.com/p4tin/goaws/app/gosqs/gosqs.go:581 +0x150e
net/http.HandlerFunc.ServeHTTP(0x783760, 0x7c75e0, 0xc000138620, 0xc000137c00)
	/usr/local/go/src/net/http/server.go:1964 +0x44
github.com/p4tin/goaws/app/router.actionHandler(0x7c75e0, 0xc000138620, 0xc000137c00)
	/go/src/github.com/p4tin/goaws/app/router/router.go:62 +0xb6
net/http.HandlerFunc.ServeHTTP(0x7837c0, 0x7c75e0, 0xc000138620, 0xc000137c00)
	/usr/local/go/src/net/http/server.go:1964 +0x44
github.com/p4tin/goaws/vendor/github.com/gorilla/mux.(*Router).ServeHTTP(0xc0000572c0, 0x7c75e0, 0xc000138620, 0xc000137c00)
	/go/src/github.com/p4tin/goaws/vendor/github.com/gorilla/mux/mux.go:150 +0xf1
net/http.serverHandler.ServeHTTP(0xc000079d40, 0x7c75e0, 0xc000138620, 0xc000137a00)
	/usr/local/go/src/net/http/server.go:2741 +0xab
net/http.(*conn).serve(0xc00015e0a0, 0x7c78e0, 0xc000053380)
	/usr/local/go/src/net/http/server.go:1847 +0x646
created by net/http.(*Server).Serve
	/usr/local/go/src/net/http/server.go:2851 +0x2f5

Basically what is happening, first we iterate over messages with index i and when we delete we override the messages. But once we delete an element from messages the index i from the outer loop is already outdated and we can't rely on it.
The solution is to switch loops, iterate over deleteEntries first and then start looking for message to be deleted, so index i always will be actual.