[BP: 6.6.x] The socket pool manager for REST calls process should be further down the start order

Clone of MB-53811 to track backporting to 6.6.6.

The socket pool manager process for REST calls, rest_lhttpc_pool is fairly high up the start order of the ns_server_nodes_sup process. However, it shouldn't need to be as this process should be able to be restarted relatively independently. We should move it down the start order or perhaps add it to a different one_for_one supervision tree.