peer: Minor function definition order cleanup.
peer: Minor function definition order cleanup.

This rearranges some of the function definitions that pertain to initial
peer version negotiation and bringup so they are more consistent with
the preferred order used throughout the codebase.  In particular, the
functions are defined before they're first used and generally as close
as possible to the first use when they're defined in the same file.

There are no functional changes.

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