Scale-out storage: IMAP CONDSTORE raises the mark, and the two-node check (ST-7, test 11)

A FETCH with CHANGEDSINCE presents its mod-sequence to the read scope, so
a replica must have that change before it answers, as a JMAP sinceState
already did. replica_cluster_tests covers test 11: with the replica's
replay paused, a write on one node is read back through a second store
with its own marks, sharing through Redis.

Composite stores nest store futures deeply enough to pass rustc's default
query depth once both postgres and redis are compiled in, so the server
crates raise their recursion limit.

tests/src/store/mod.rs

@@ -14,6 +14,8 @@ pub mod registry;
 pub mod replica; // inbuxa: read replicas
 #[cfg(feature = "mysql")]
 pub mod replica_mysql; // inbuxa: read replicas on MySQL
+#[cfg(all(feature = "postgres", feature = "redis"))]
+pub mod replica_cluster; // inbuxa: read replicas across nodes
 pub mod scaleout; // inbuxa: scale-out storage
 #[cfg(any(feature = "postgres", feature = "mysql"))]
 pub mod sql_timeout;