Refactor Restore()

This commit refactors out the complexity of downloading ordered WAL
files in parallel to a type called `WALDownloader`. This makes it
easier to test the restore separately from the download.

testdata/generation-time-bounds/ok/Makefile

@@ -0,0 +1,8 @@
+.PHONY: default
+default:
+	TZ=UTC touch -t 200001010000 generations/0000000000000000/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001020000 generations/0000000000000000/snapshots/00000001.snapshot.lz4
+	TZ=UTC touch -t 200001010000 generations/0000000000000000/wal/00000000/00000000.wal.lz4
+	TZ=UTC touch -t 200001020000 generations/0000000000000000/wal/00000000/00000001.wal.lz4
+	TZ=UTC touch -t 200001030000 generations/0000000000000000/wal/00000001/00000000.wal.lz4
+

testdata/generation-time-bounds/snapshots-only/Makefile

@@ -0,0 +1,5 @@
+.PHONY: default
+default:
+	TZ=UTC touch -t 200001010000 generations/0000000000000000/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001020000 generations/0000000000000000/snapshots/00000001.snapshot.lz4
+