Initial pass at updating to btcnet.

This change modifies the params struct to embed a *btcnet.Params,
removing the old parameter fields that are handled by the btcnet
package.

Hardcoded network checks have also been removed in favor of modifying
behavior based on the current active net's parameters.

Not all library packages, notable btcutil and btcchain, have been
updated to use btcnet yet, but with this change, each package can be
updated one at a time since the active net's btcnet.Params are
available at each callsite.

ok @davecgh