watcher: Write per-group instance status, merge into global one

Each per-group watcher process writes its own instance status file. Once
that's done it tries to acquire an exclusive lock on the global file and
will proceed to read all status file, merging them based on each file's
mtime. If an instance is moved to another group, the newer status will
supersede that of an older file which hasn't yet been updated.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>