Major storage format rewrite

Major changes and new concepts:

Simplify Repository to a value-store
------------------------------------

Move the responsibility of tracking relation metadata, like which relations
exist and what are their sizes, from Repository to a new module,
pgdatadir_mapping.rs. The interface to Repository is now a simple key-value
PUT/GET operations.

It's still not any old key-value store though. A Repository is still
responsible from handling branching, and every GET operation comes with
an LSN.

Key
---

The key to the Repository key-value store is a Key struct, which consists
of a few integer fields. It's wide enough to store a full RelFileNode,
fork and block number, and to distinguish those from metadata keys.

See pgdatadir_mapping.rs for how relation blocks and metadata keys are
mapped to the Key struct.

Store arbitrary key-ranges in the layer files
---------------------------------------------

The concept of a "segment" is gone. Each layer file can store an arbitrary
range of Keys.

TODO:

- Deleting keys, to reclaim space. This isn't visible to Postgres, dropping
  or truncating a relation works as you would expect if you look at it from
  the compute node. If you drop a relation, for example, the relation is
  removed from the metadata entry, so that it appears to be gone. However,
  the layered repository implementation never reclaims the storage.

- Tracking "logical database size", for disk space quotas. That ought to
  be reimplemented now in pgdatadir_mapping.rs, or perhaps in walingest.rs.

- LSM compaction. The logic for checkpointing and creating image layers is
  very dumb. AFAIK the *read* code could deal with a full-fledged LSM tree
  now consisting of the delta and image layers. But there's no code to
  take a bunch of delta layers and compact them, and the heuristics for
  when to create image layers is pretty dumb.

- The code to track the layers is inefficient. All layers are just stored in
  a vector, and whenever we need to find a layer, we do a linear search in
  it.

postgres_ffi/src/pg_constants.rs

@@ -24,6 +24,9 @@ pub const VISIBILITYMAP_FORKNUM: u8 = 2;
 pub const INIT_FORKNUM: u8 = 3;
 
 // From storage_xlog.h
+pub const XLOG_SMGR_CREATE: u8 = 0x10;
+pub const XLOG_SMGR_TRUNCATE: u8 = 0x20;
+
 pub const SMGR_TRUNCATE_HEAP: u32 = 0x0001;
 pub const SMGR_TRUNCATE_VM: u32 = 0x0002;
 pub const SMGR_TRUNCATE_FSM: u32 = 0x0004;
@@ -113,7 +116,6 @@ pub const XACT_XINFO_HAS_TWOPHASE: u32 = 1u32 << 4;
 // From pg_control.h and rmgrlist.h
 pub const XLOG_NEXTOID: u8 = 0x30;
 pub const XLOG_SWITCH: u8 = 0x40;
-pub const XLOG_SMGR_TRUNCATE: u8 = 0x20;
 pub const XLOG_FPI_FOR_HINT: u8 = 0xA0;
 pub const XLOG_FPI: u8 = 0xB0;
 pub const DB_SHUTDOWNED: u32 = 1;