node: add a lock to spoolreader

As cnu noticed on IRC, no locking is done in the PoolReader. As
commit 42d37e8633616d13d1f7c52c9ec67c4bb546804d adds it when
writing, it is quite cheap to also add it on reading. It nicely
solves the common use case of asyncd and async on the same host.

It might even work on flock-compliant NFS shares.