watchtower/wtdb: add ClientDB

This commit adds the full bbolt-backed client database as well as a set
of unit tests to assert that it exactly implements the same behavior as
the mock ClientDB.

watchtower/wtdb/client_chan_summary.go

@@ -1,18 +1,11 @@
 package wtdb
 
 import (
-	"errors"
 	"io"
 
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
-var (
-	// ErrChannelAlreadyRegistered signals a duplicate attempt to
-	// register a channel with the client database.
-	ErrChannelAlreadyRegistered = errors.New("channel already registered")
-)
-
 // ChannelSummaries is a map for a given channel id to it's ClientChanSummary.
 type ChannelSummaries map[lnwire.ChannelID]ClientChanSummary