Fix shutdown persistence issue in Seednode

[ghost]

Fix issue raised by @Emzy in Keybase ops. Seednode v1.6.0 was not persisting on shutdown.

#5280 introduced a guard in PersistenceManager to not persist at shutdown if a valid app start was not seen.
Initialize PersistenceManager in Seednode app so that the expected persist on shutdown can occur.

@chimp1984 plz check.

[edit] per @chimp1984 suggestion below, moved the init call to AppSetupWithP2P.onBasicServicesInitialized

[chimp1984]

@jmacxx Ah thanks for spotting that!
I think its better to do it in 'AppSetupWithP2P.onBasicServicesInitialized'. There are some other headless apps using those old startup code as well I guess.

[chimp1984]

utACK

[Emzy]

testedACK
Works on my seednodes

Apr-03 16:57:35.996 [SeedNodeMain] WARN  b.n.p.n.TorNetworkNode: We got called shutDown again and ignore it. 
Apr-03 16:57:35.998 [SeedNodeMain] INFO  b.n.p.n.NetworkNode: Shutdown 16 connections 
Apr-03 16:57:36.044 [SeedNodeMain] INFO  b.c.p.PersistenceManager: Start flushAllDataToDisk 
...
Apr-03 16:57:36.084 [SeedNodeMain] INFO  b.n.p.n.NetworkNode: Shutdown completed with all connections closed 
Apr-03 16:57:36.086 [SeedNodeMain] WARN  b.c.p.PersistenceManager: We got flushAllDataToDisk called again. This can happen in some
 rare cases. We ignore the repeated call. 
Apr-03 16:57:36.090 [SeedNodeMain] INFO  b.c.p.PersistenceManager: flushAllDataToDisk completed 

[ripcurlx]

utACK based on #5388 (comment)