Change client_id in taskmanager to key_id

[badb]

Previously clients were identified by uuid called client_id. Then we have added identification by public key (called key_id). Task manager and tasks instances still uses client_id (for example to identify if a client has counted a subtask in that task already). This should be replace with a new method.

[badb]

It's a part of bigger task of replacing client_id with key_id in all Golem classes: https://github.com/imapp-pl/golem/issues/41