Acquire an exclusive OS advisory lock on a sidecar file beside the
database path (<db_path>.lock) before opening the store, so a second
nunciod pointed at the same database fails fast with an actionable
error instead of both instances writing to the same SQLite file.
Previously only an incidental gRPC port-bind collision offered any
protection against this.
Uses fs4 (flock on Unix, LockFileEx on Windows) since std has no
portable file-lock primitive. The guard is held for the process
lifetime and dropped at clean shutdown, releasing the lock; the OS
also releases it automatically if the process dies uncleanly, so
there is no PID-file staleness to reason about.
Refs #291
James Maes