chore(all): memory and thread safe storage

- settings: get filter choices from storage for settings validation
- updater: update servers to the storage
- storage: minimal deep copying and data duplication
- storage: add merged servers mutex for thread safety
- connection: filter servers in storage
- formatter: format servers to Markdown in storage
- PIA: get server by name from storage directly
- Updater: get servers count from storage directly
- Updater: equality check done in storage, fix #882

internal/provider/common/storage.go

@@ -0,0 +1,12 @@
+package common
+
+import (
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+type Storage interface {
+	FilterServers(provider string, selection settings.ServerSelection) (
+		servers []models.Server, err error)
+	GetServerByName(provider, name string) (server models.Server, ok bool)
+}