Add persistent message map

Resolves #541

vendor/github.com/philippgille/gokv/store.go

@@ -0,0 +1,40 @@
+package gokv
+
+// Store is an abstraction for different key-value store implementations.
+// A store must be able to store, retrieve and delete key-value pairs,
+// with the key being a string and the value being any Go interface{}.
+type Store interface {
+	// Set stores the given value for the given key.
+	// The implementation automatically marshalls the value.
+	// The marshalling format depends on the implementation. It can be JSON, gob etc.
+	// The key must not be "" and the value must not be nil.
+	Set(k string, v interface{}) error
+	// Get retrieves the value for the given key.
+	// The implementation automatically unmarshalls the value.
+	// The unmarshalling source depends on the implementation. It can be JSON, gob etc.
+	// The automatic unmarshalling requires a pointer to an object of the correct type
+	// being passed as parameter.
+	// In case of a struct the Get method will populate the fields of the object
+	// that the passed pointer points to with the values of the retrieved object's values.
+	// If no value is found it returns (false, nil).
+	// The key must not be "" and the pointer must not be nil.
+	Get(k string, v interface{}) (found bool, err error)
+	// Delete deletes the stored value for the given key.
+	// Deleting a non-existing key-value pair does NOT lead to an error.
+	// The key must not be "".
+	Delete(k string) error
+	// Close must be called when the work with the key-value store is done.
+	// Most (if not all) implementations are meant to be used long-lived,
+	// so only call Close() at the very end.
+	// Depending on the store implementation it might do one or more of the following:
+	// Make sure all pending updates make their way to disk,
+	// finish open transactions,
+	// close the file handle to an embedded DB,
+	// close the connection to the DB server,
+	// release any open resources,
+	// etc.
+	// Some implementation might not need the store to be closed,
+	// but as long as you work with the gokv.Store interface you never know which implementation
+	// is passed to your method, so you should always call it.
+	Close() error
+}