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/tower.go

@@ -1,7 +1,6 @@
 package wtdb
 
 import (
-	"errors"
 	"io"
 	"net"
 
@@ -9,12 +8,6 @@ import (
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
-var (
-	// ErrTowerNotFound signals that the target tower was not found in the
-	// database.
-	ErrTowerNotFound = errors.New("tower not found")
-)
-
 // TowerID is a unique 64-bit identifier allocated to each unique watchtower.
 // This allows the client to conserve on-disk space by not needing to always
 // reference towers by their pubkey.