slapproxy: Make rows unique for forwarded requests

Avoid bloating the proxy database due to each instance processing
adding the same entries for forwarded requests as duplicate rows
again and again, leading to unbounded growth.

slapos/proxy/schema.sql

@@ -54,5 +54,6 @@ CREATE TABLE IF NOT EXISTS partition_network%(version)s (
 
 CREATE TABLE IF NOT EXISTS forwarded_partition_request%(version)s (
   partition_reference VARCHAR(255), -- a.k.a source_instance_id
-  master_url VARCHAR(255)
+  master_url VARCHAR(255),
+  CONSTRAINT uniq PRIMARY KEY (partition_reference, master_url)
 );