Update index & offset encoding

Previously, the index & offsets were encoded as 8-character hex
strings, however, this limits the maximum value to a `uint32`. This
is normally not an issue, however, indices could go over the maximum
value of 4 billion over time and the offset could exceed this value
for an especially large WAL update. For safety, these encodings have
been updated to 16-character hex encodings.

testdata/max-snapshot-index/ok/Makefile

@@ -1,6 +1,6 @@
 .PHONY: default
 default:
-	TZ=UTC touch -ct 200001020000 generations/0000000000000000/snapshots/00000000.snapshot.lz4
-	TZ=UTC touch -ct 200001010000 generations/0000000000000001/snapshots/00000000.snapshot.lz4
-	TZ=UTC touch -ct 200001030000 generations/0000000000000001/snapshots/00000001.snapshot.lz4
-	TZ=UTC touch -ct 200001010000 generations/0000000000000002/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -ct 200001020000 generations/0000000000000000/snapshots/0000000000000000.snapshot.lz4
+	TZ=UTC touch -ct 200001010000 generations/0000000000000001/snapshots/0000000000000000.snapshot.lz4
+	TZ=UTC touch -ct 200001030000 generations/0000000000000001/snapshots/0000000000000001.snapshot.lz4
+	TZ=UTC touch -ct 200001010000 generations/0000000000000002/snapshots/0000000000000000.snapshot.lz4