Use Unix socket to pass IMAP updates from maddyctl to daemon

There is abstraction 'updates pipe' defined for future use with
configuration involving IMAP data replication (e.g. multiple nodes with
maddy instances + PostgreSQL replicas + S3 bucket for messages).

However, for the case of local SQLite3 DB, limited UDS-based
implementation is provided. It solves the problem of maddyctl not being
able to tell the server about modifications it makes. Alternative to
this approach would be to have server actually perform operations and
maddyctl being a dumb API client, but this requires a lot more complex
IPC interface and will not work when the server is down.

internal/updatepipe/backend.go

@@ -0,0 +1,27 @@
+package updatepipe
+
+type BackendMode int
+
+const (
+	// ModeReplicate configures backend to both send and receive updates over
+	// the pipe.
+	ModeReplicate BackendMode = iota
+
+	// ModePush configures backend to send updates over the pipe only.
+	//
+	// If EnableUpdatePipe(ModePush) is called for backend, its Updates()
+	// channel will never receive any updates.
+	ModePush BackendMode = iota
+)
+
+// The Backend interface is implemented by storage backends that support both
+// updates serialization using the internal updatepipe.P implementation.
+// To activate this implementation, EnableUpdatePipe should be called.
+type Backend interface {
+	// EnableUpdatePipe enables the internal update pipe implementation.
+	// The mode argument selects the pipe behavior. EnableUpdatePipe must be
+	// called before the first call to the Updates() method.
+	//
+	// This method is idempotent. All calls after a successful one do nothing.
+	EnableUpdatePipe(mode BackendMode) error
+}