Introduce first version of tenant migraiton between pageservers

This patch includes attach/detach http endpoints in pageservers. Some
changes in callmemaybe handling inside safekeeper and an integrational
test to check migration with and without load. There are still some
rough edges that will be addressed in follow up patches

walkeeper/src/handler.rs

@@ -30,6 +30,7 @@ pub struct SafekeeperPostgresHandler {
     pub ztenantid: Option<ZTenantId>,
     pub ztimelineid: Option<ZTimelineId>,
     pub timeline: Option<Arc<Timeline>>,
+    pageserver_connstr: Option<String>,
     //sender to communicate with callmemaybe thread
     pub tx: UnboundedSender<CallmeEvent>,
 }
@@ -88,6 +89,8 @@ impl postgres_backend::Handler for SafekeeperPostgresHandler {
                 self.appname = Some(app_name.clone());
             }
 
+            self.pageserver_connstr = params.get("pageserver_connstr").cloned();
+
             Ok(())
         } else {
             bail!("Walkeeper received unexpected initial message: {:?}", sm);
@@ -127,7 +130,7 @@ impl postgres_backend::Handler for SafekeeperPostgresHandler {
             }
             SafekeeperPostgresCommand::StartReplication { start_lsn } => {
                 ReplicationConn::new(pgb)
-                    .run(self, pgb, start_lsn)
+                    .run(self, pgb, start_lsn, self.pageserver_connstr.clone())
                     .context("failed to run ReplicationConn")?;
             }
             SafekeeperPostgresCommand::IdentifySystem => {
@@ -149,6 +152,7 @@ impl SafekeeperPostgresHandler {
             ztenantid: None,
             ztimelineid: None,
             timeline: None,
+            pageserver_connstr: None,
             tx,
         }
     }