blockEventQueue is an ordered queue for block events sent from a
FilteredChainView. The two types of possible block events are
connected/new blocks, and disconencted/stale blocks. The
blockEventQueue keeps the order of these events intact, while
still being non-blocking. This is important in order for the
chainView's call to onFilteredBlockConnected/Disconnected to not
get blocked, and for the consumer of the block events to always
get the events in the correct order.
