MM-32893: Refactor the getting of DB handle (#16943)

* MM-32893: Refactor the getting of DB handle

We add a method in *SqlStore that returns the correct DB handle
depending on the context.

https://mattermost.atlassian.net/browse/MM-32893

```release-note
NONE
```

* fix receiver name

store/sqlstore/context.go

@@ -5,6 +5,8 @@ package sqlstore
 
 import (
 	"context"
+
+	"github.com/mattermost/gorp"
 )
 
 // storeContextKey is the base type for all context keys for the store.
@@ -32,3 +34,11 @@ func hasMaster(ctx context.Context) bool {
 	}
 	return false
 }
+
+// DBFromContext is a helper utility that returns the DB handle from a given context.
+func (ss *SqlStore) DBFromContext(ctx context.Context) *gorp.DbMap {
+	if hasMaster(ctx) {
+		return ss.GetMaster()
+	}
+	return ss.GetReplica()
+}