refactor: drop vestigial client param from _consume

[lesnik512]

Closes #65.

TimersSubscriber.start() passed self._client into _consume as a positional argument, and _consume used it once, for the startup ping() that sets start_signal. The loop's Redis I/O already goes through TimerStore, which reads ConnectionState.client on every call. _consume now takes only start_signal and pings through the existing _client property. This finishes what #43 did for _get_msgs and _claim_and_consume.

#65 raised a reconnect concern: binding the client at start could keep a stale client. That doesn't apply. The bound client was only ever used for the one ping before the loop starts, and nothing rebinds ConnectionState._client while the broker runs (connect() returns the caller-owned client, disconnect() does nothing, and only the test broker's _patch_broker swaps it, before any loop runs). So behavior is unchanged and no comment about binding is needed.

add_task is called with func_kwargs= by keyword since func_args defaults to None.

Verification:

• just lint-ci passes.
• Full suite: 168 passed against Redis 8, no skips, no test changes.