Reject pending fetch when block tracker is stopped

[Gudahtt]

The #pendingLatestBlock private instance variable was added to track a pending request for the latest block. However, if _isRunning was true when this was setup, and the polling loop is stopped before the fetch request was initiated, #pendingLatestBlock would stay unresolved forever.

We should reject #pendingLatestBlock if the polling loop is stopped.

Steps to reproduce:

• Start the block tracker
• Wait for the first block update to resolve
• Immediately after, call getLatestBlock
• Immediately after, stop the block tracker.
• The call to getLatestBlock would then never resolve