query.Peer is used for downloading blocks out of order during headers
first download.  Methods SubscribeRecvMsg() and OnDisconnect() are added
to abide by the interface.

ConnectedPeers returns all the currently connected peers.  This is used
to provide the query.WorkManager with all the currently connected peers
from the netsync package.

checkpointedBlocksQuery is a helper to create []*query.Request which can
be passed off to query.Workmanager to query for wire.Messages to
multiple peers.  This is useful for downloading blocks out of order from
multiple peers during ibd.

handleBlockMsg used to check that the block header is both valid and
then process the blocks as they come in.  It's now refactored so that
it also handles blocks that are not in order.  For out of order block
downloads handleBlockMsg would mark the block as an orphan but it's now
refactored to handle those cases.

Whenever a block that's not the next from the chain tip is received,
it's now temporarily stored in memory until the next block from the
chain tip is received.  And then all the blocks that are in sequence are
processed.

peerSubscription is added to Manager which will allow it subscribers to
receive peers through the channel whenever the Manager is aware of a new
peer that it's been connected to.  This is useful to alert
query.Workmanager that a new peer that's been connected to is eligible
to download blocks from.

ConnectedPeers returns all the currently connected peers and any new
peer that's additionally connected through the returned channel.  This
method is required for query.Workmanager as it needs ot receive peers
that it can request blocks from.

The blocks that were requested from headers are now sent over to
query.Workmanager where it will rank peers based on their speed and
request blocks from them accordingly.  This allows for quicker block
downloads as:
1: Workmanager will prioritize faster peers.
2: Workmanager is able to ask from multiple peers.

Storing block happens before the block validation is done and this can
be a bottleneck on computers with slow disks.  Allowing for concurrent
block storage saves time as the disk operation can be done in parallel
with the cpu operations of verifying the block.

headers-first block download

Resetting the requestedBlocks state in headersFirst is problematic since
we may be banning peers that are still good.