support tenants

this patch adds support for tenants. This touches mostly pageserver.
Directory layout on disk is changed to contain new layer of indirection.
Now path to particular repository has the following structure: <pageserver workdir>/tenants/<tenant
id>. Tenant id has the same format as timeline id. Tenant id is included in
pageserver commands when needed. Also new commands are available in
pageserver: tenant_list, tenant_create. This is also reflected CLI.
During init default tenant is created and it's id is saved in CLI config,
so following commands can use it without extra options. Tenant id is also included in
compute postgres configuration, so it can be passed via ServerInfo to
safekeeper and in connection string to pageserver.
For more info see docs/multitenancy.md.

Cargo.lock

@@ -268,6 +268,7 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "bytes",
+ "hex",
  "lazy_static",
  "nix",
  "pageserver",
@@ -674,6 +675,9 @@ name = "hex"
 version = "0.4.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7f24254aa9a54b5c858eaee2f5bccdb46aaf0e486a595ed5fd8f86ba55232a70"
+dependencies = [
+ "serde",
+]
 
 [[package]]
 name = "hex-literal"