MM-14143 config cleanup final (#10374)

* TestGetLicenseFileFromDisk: avoid using fileutils.FindConfigFile

* config: abstract config-related file access, extend memory store

* simplify config validate to avoid file knowledge

* fix relative file tests

* cluster: fix ConfigChanged event

The old and new configurations were swapped when notifying the enterprise code of configuration changes, creating needless instability in propagating config updates across a cluster.

* config/database: ignore duplicates

* test cleanup

* remove unnecessary Save() in test

config/store.go

@@ -32,6 +32,19 @@ type Store interface {
 	// RemoveListener removes a callback function using an id returned from AddListener.
 	RemoveListener(id string)
 
+	// GetFile fetches the contents of a previously persisted configuration file.
+	// If no such file exists, an empty byte array will be returned without error.
+	GetFile(name string) ([]byte, error)
+
+	// SetFile sets or replaces the contents of a configuration file.
+	SetFile(name string, data []byte) error
+
+	// HasFile returns true if the given file was previously persisted.
+	HasFile(name string) (bool, error)
+
+	// RemoveFile removes a previously persisted configuration file.
+	RemoveFile(name string) error
+
 	// String describes the backing store for the config.
 	String() string