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/mocks_generate_test.go

@@ -0,0 +1,5 @@
+package common
+
+// Exceptionally, the storage mock is exported since it is used by all
+// provider subpackages tests, and it reduces test code duplication a lot.
+//go:generate mockgen -destination=mocks.go -package $GOPACKAGE . Storage