wtdb+wtclient: segregate session-key-index reservations by blob-type

This commit introduces a change in the key format used to reserve/lookup
session-key-indexes. Currently the reservations are stored under the
tower id, however this creates issues when multiple clients are using
the same database since only one reservation is permitted per tower.

We fix this by appending the blob type to the session-key-index locator.
This allows multiple clients to reserve keys for the same tower, but
still limits each client to one outstanding reservation. The changes are
made in a way such that we fall back to the legacy format if the a
reservation under the new format is not found, but only if the blob type
matches blob.TypeAltruistCommit, which is so far the only actively
deployed blob type.

watchtower/wtclient/interface.go

@@ -7,6 +7,7 @@ import (
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/tor"
+	"github.com/lightningnetwork/lnd/watchtower/blob"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb"
 	"github.com/lightningnetwork/lnd/watchtower/wtserver"
 )
@@ -44,12 +45,12 @@ type DB interface {
 	ListTowers() ([]*wtdb.Tower, error)
 
 	// NextSessionKeyIndex reserves a new session key derivation index for a
-	// particular tower id. The index is reserved for that tower until
-	// CreateClientSession is invoked for that tower and index, at which
-	// point a new index for that tower can be reserved. Multiple calls to
-	// this method before CreateClientSession is invoked should return the
-	// same index.
-	NextSessionKeyIndex(wtdb.TowerID) (uint32, error)
+	// particular tower id and blob type. The index is reserved for that
+	// (tower, blob type) pair until CreateClientSession is invoked for that
+	// tower and index, at which point a new index for that tower can be
+	// reserved. Multiple calls to this method before CreateClientSession is
+	// invoked should return the same index.
+	NextSessionKeyIndex(wtdb.TowerID, blob.Type) (uint32, error)
 
 	// CreateClientSession saves a newly negotiated client session to the
 	// client's database. This enables the session to be used across