log: avoid buffer overflow in vlog

log: avoid buffer overflow in vlog

`vlog()` can not handle log messages longer than 4096 bytes. However, the
message generated in `store_restart_layout()` is likely to exceed this
as it contains a long JSON string.

This has caused a few SEGFAULTS during restarts for me when running with
`-d all`.

Fix this by truncating the message to 4096 bytes and punching in a newline at
the end.