Make the `top_level` parameter configurable.
Support sys messages in hanoidb_merger processes
Don’t open exclusive when re-opening a merge file
Fix min/max level for recovery

Previous commit f0d24894c5f9aa14e58ab3e18b6e2ef2a88024cf
introduced a bug in recovery code.
Fix up the ensure_started code
Remove some lock contention when fsync’ing
Fix crash in recovery code
Update travis to include 17