Begin update to use btcnet.Params.

This is an intial pass at converting the btcwallet and deps codebases
to pass a network by their parameters, rather than by a magic number
to identify the network.  The parameters in params.go have been
updated to embed a *btcnet.Params, and all previous uses of cfg.Net()
have been replaced with activeNet.{Params,Net} (where activeNet is
the global var for the active network).

Although dependancy packages have not yet been updated from using
btcwire.BitcoinNet to btcnet.Params, the parameters are now accessible
at all callsites, and individual packages can be updated to use btcnet
without requiring updates in each external btc* package at once.

While here, the exported API for btcwallet internal library packages
(txstore and wallet) have been updated to pass full network parameters
rather than the btcwire definition of a network.