Invoice subscription: safety improvements

[kiwiidb]

Fixes #312 , fixes #313 This PR changes the behaviour of the invoice subscription routine in case of an error:

• If the database call at startup time fails, we return an error which will crash LNDhub and force a restart.

• If the Recv() call fails (probably because LND is down), then we will also force LNDhub to restart, so it can try to reconnect to LND.

  Other than that

• It introduces a database timeout config option, with a default value of 1 minutes (up from 5 seconds)
• It introduces a safety buffer of 14 hours to make sure we absolutely did not miss any invoices which might have been paid while LNDhub was down.

[reneaaron]

@kiwiidb Should we also add an index for the invoice subscription query, too? It's only a matter of time until this query takes longer than the timeout.

explain analyze select * from public.invoices where invoices.settled_at IS NULL AND invoices.add_index IS NOT NULL AND invoices.expires_at >= now() order by invoices.id ASC

leads to some really awful query plan as there are no indexes to be used currently.

I think we would need to add a combined column index on the used columns to speed this up.

[bumi]

@reneaaron of course :) indexes need to be pimped