blockmanager: Simplify dynamic peer height updates.
blockmanager: Simplify dynamic peer height updates.

This simplifies the code that updates the latest known block height for
peer based on blocks received from them to use the fact the height is in
the header.  The previous code was originally from upstream where the
height was not available in the header.

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