feat(distlock): refactor into a standalone package

pkg/distlock/doc.go

@@ -0,0 +1,7 @@
+// Package distlock provides distributed, automatically renewed leases across independent nodes.
+//
+// A Cluster must return every node in the lock group, including temporarily unavailable nodes, because every node
+// counts toward quorum. Changing the node set is unsafe if a new quorum can be disjoint from an earlier quorum while
+// leases acquired from the earlier node set may still be valid. Callers must stop protected work when the context
+// returned by Lease.Context is done.
+package distlock