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/models/filters.go

@@ -0,0 +1,10 @@
+package models
+
+type FilterChoices struct {
+	Countries []string
+	Regions   []string
+	Cities    []string
+	ISPs      []string
+	Names     []string
+	Hostnames []string
+}