Putting the test code in the same package makes it easier for forks
since they don't have to change the import paths as much and it also
gets rid of the need for internal_test.go to bridge.
This same thing should probably be done for the majority of the code
base.
This implements the wire protocol encoding portion of a new
sendheaders message as described by BIP0130. It purpose is to request
that a peer sends header commands instead of inv commands when
announcing new blocks. This includes a protocol version bump to 70012
and a wire version bump to 0.4.0.
Note that this does not implement logic to handle the command in btcd,
rather it only makes the command available at the wire protocol level.
A future commit which honors the command and therefore provides full
BIP0130 support is still required.
