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/storage/storage.go

@@ -2,11 +2,14 @@
 package storage
 
 import (
+	"sync"
+
 	"github.com/qdm12/gluetun/internal/models"
 )
 
 type Storage struct {
 	mergedServers models.AllServers
+	mergedMutex   sync.RWMutex
 	// this is stored in memory to avoid re-parsing
 	// the embedded JSON file on every call to the
 	// SyncServers method.