memory provider: load users from a dump file

The `memory` provider can load users from a dump obtained using the
`dumpdata` REST API. This dump file can be configured using the
dataprovider `name` configuration key. It will be loaded at startup
and can be reloaded on demand using a `SIGHUP` on Unix based systems
and a `paramchange` request to the running service on Windows.

Fixes #66

dataprovider/sqlite.go

@@ -105,3 +105,7 @@ func (p SQLiteProvider) getUsers(limit int, offset int, order string, username s
 func (p SQLiteProvider) close() error {
 	return p.dbHandle.Close()
 }
+
+func (p SQLiteProvider) reloadConfig() error {
+	return nil
+}