Refactor CLI and CLI<->pageserver interfaces to support remote pageserver

This patch started as an effort to support CLI working against remote
pageserver, but turned into a pretty big refactoring.

* CLI now does not look into repository files directly. New commands
'branch_create' and 'identify_system' were introduced into page_service to
support that.
* Branch management that was scattered between local_env and
zenith/main.rs is moved into pageserver/branches.rs. That code could better fit
in Repository/Timeline impl, but I'll leave that for a different patch.
* All tests-related code from local_env went into integration_tests/src/lib.rs as an
extension to PostgresNode trait.
* Paths-generating functions were concentrated around corresponding config
types (LocalEnv and PageserverConf).

walkeeper/src/lib.rs

@@ -3,7 +3,7 @@ use std::net::SocketAddr;
 use std::path::PathBuf;
 use std::time::Duration;
 
-mod pq_protocol;
+pub mod pq_protocol;
 pub mod s3_offload;
 pub mod wal_service;