Change behavior of ComputeControlPlane::new_node() (#235)

Previously, transaction commit could happen regardless of whether
pageserver has caught up or not. This patch aims to fix that.

There are two notable changes:

1. ComputeControlPlane::new_node() now sets the
`synchronous_standby_names = 'pageserver'` parameter to delay
transaction commit until pageserver acting as a standby has
fetched and ack'd a relevant portion of WAL.

2. pageserver now has to:
    - Specify the `application_name = pageserver` which matches the
    one in `synchronous_standby_names`.
    - Properly reply with the ack'd LSNs.

This means that some tests don't need sleeps anymore.

TODO: We should probably make this behavior configurable.

Fixes #187.

control_plane/src/compute.rs

@@ -151,8 +151,11 @@ impl ComputeControlPlane {
         node.append_conf(
             "postgresql.conf",
             format!(
-                "shared_preload_libraries = zenith\n\
-                zenith.callmemaybe_connstring = '{}'\n", // FIXME escaping
+                concat!(
+                    "shared_preload_libraries = zenith\n",
+                    "synchronous_standby_names = 'pageserver'\n", // TODO: add a new function arg?
+                    "zenith.callmemaybe_connstring = '{}'\n", // FIXME escaping
+                ),
                 node.connstr()
             )
             .as_str(),