server: ensure the replay database is stored under the graph+network namespace
server: ensure the replay database is stored under the graph+network namespace

In this commit we modify the storage location of the sphinx replay
database to be under the precise network, and not only the graph sub
directory. Before this commit, due to the usage of filepath.Dir(), the
db would lie under /graph/, rather than say, /graph/simnet.

LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd