contractcourt/contract_resolvers: fix subscribe preimage race
contractcourt/contract_resolvers: fix subscribe preimage race

This commit fixes a potential race condition within the
IncomingContestResolver, that could cause us to miss a
preimage that was delivered in time.

Currently we query the db for the preimage, and then
subscribe for notifications. This permits the following
ordering of events:
 - query for preimage, returns nothing
 - preimage is added and delivered to subscribers
 - subscribe to preimages
 - preimage never comes through!!

We fix this by reordering to subscribe for preimages and
then query just in case it already exists. The effect is
that the query will always return a valid read of the
preimages that are currently queued for delivery.

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