channeldb: create new channeldb package, update lnwallet to use new API

* Initial draft of brain dump of chandler. Nothing yet set in stone.
* Will most likely move the storage of all structs to a more “column”
oriented approach. Such that, small updates like incrementing the total
satoshi sent don’t result in the entire struct being serialized and
written.
* Some skeleton structs for other possible data we might want to store
are also included.
* Seem valuable to record as much data as possible for record keeping,
visualization, debugging, etc. Will need to set up a time+space+dirty
cache to ensure performance isn’t impacted too much.

lnwallet/wallet_test.go

@@ -591,10 +591,7 @@ func clearWalletState(w *LightningWallet) error {
 	w.nextFundingID = 0
 	w.fundingLimbo = make(map[uint64]*ChannelReservation)
 	w.wallet.ResetLockedOutpoints()
-	return w.channelDB.namespace.Update(func(tx walletdb.Tx) error {
-		rootBucket := tx.RootBucket()
-		return rootBucket.DeleteBucket(openChannelBucket)
-	})
+	return w.channelDB.Wipe()
 }
 
 // TODO(roasbeef): why is wallet so slow to create+open?