Improve attachment memory allocation (#105)

* Improve attachment memory allocation

* added explicit garbage collection

* redundant GC

* added GCs in attachment processor and a FreeOSMemory helper

* added changelog

* linting

* Remove commented call to debug.FreeOSMemory

Co-authored-by: marin thiercelin <marin.thiercelin@pm.me>
Co-authored-by: marinthiercelin <marin.thiercelin@epfl.ch>

helper/mobile.go

@@ -3,6 +3,7 @@ package helper
 import (
 	"encoding/json"
 	goerrors "errors"
+	"runtime/debug"
 
 	"github.com/ProtonMail/gopenpgp/v2/crypto"
 	"github.com/pkg/errors"
@@ -124,3 +125,10 @@ func EncryptSignBinaryDetachedMobile(
 		EncryptedSignatureArmored: encryptedSignature,
 	}, nil
 }
+
+// FreeOSMemory can be used to explicitly
+// call the garbage collector and
+// return the unused memory to the OS.
+func FreeOSMemory() {
+	debug.FreeOSMemory()
+}