blockmanager: Fast relay checked tip blocks.
blockmanager: Fast relay checked tip blocks.

This makes use of the newly exposed notification from blockchain when a
block that intends to extend the main chain has passed all sanity and
contextual checks to relay the block the rest of the network at that
point rather than needing to wait for the more expensive connection code
to complete.

decred/dcrd,jrick/dcrd,jrick/dcrd,decred/dcrd