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.

test_runner/fixtures/utils.py

@@ -74,8 +74,5 @@ def lsn_from_hex(lsn_hex: str) -> int:
 def print_gc_result(row):
     log.info("GC duration {elapsed} ms".format_map(row))
     log.info(
-        "  REL    total: {layer_relfiles_total}, needed_by_cutoff {layer_relfiles_needed_by_cutoff}, needed_by_branches: {layer_relfiles_needed_by_branches}, not_updated: {layer_relfiles_not_updated}, needed_as_tombstone {layer_relfiles_needed_as_tombstone}, removed: {layer_relfiles_removed}, dropped: {layer_relfiles_dropped}"
-        .format_map(row))
-    log.info(
-        "  NONREL total: {layer_nonrelfiles_total}, needed_by_cutoff {layer_nonrelfiles_needed_by_cutoff}, needed_by_branches: {layer_nonrelfiles_needed_by_branches}, not_updated: {layer_nonrelfiles_not_updated}, needed_as_tombstone {layer_nonrelfiles_needed_as_tombstone}, removed: {layer_nonrelfiles_removed}, dropped: {layer_nonrelfiles_dropped}"
+        "  total: {layers_total}, needed_by_cutoff {layers_needed_by_cutoff}, needed_by_branches: {layers_needed_by_branches}, not_updated: {layers_not_updated}, removed: {layers_removed}"
         .format_map(row))