Provide ChannelStateReader#hasStates for hints of reading channel states

Currently we rely on whether unaligned checkpoint is enabled to determine whether to read recovered states during task startup, then it will block the requirements of recovery from previous unaligned states even though the current mode is aligned.

We can make `ChannelStateReader` provide the hint whether there are any channel states to be read during startup, then we will never lose any chances to recover from them.