[conformal/btcd] f309e8: Make use of the new btcdb functions.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcd
  Commit: f309e899f3e6b71a2df996e5d678f40ed85df64e
      
https://github.com/conformal/btcd/commit/f309e899f3e6b71a2df996e5d678f40ed85df64e
  Author: Dave Collins <da...@conformal.com>
  Date:   2014-01-19 (Sun, 19 Jan 2014)

  Changed paths:
    M peer.go

  Log Message:
  -----------
  Make use of the new btcdb functions.

This commit switches the handleGetHeadersMsg function to make use of the
new FetchBlockHeightBySha and FetchBlockHeaderBySha functions in btcdb.

Also, while here, nuke the header copy which is no longer required due to
the recent btcwire changes.