channeldb: make channel graph population async #6187
Description
In #6111 we significantly sped up the population of the channel graph cache. However, as is, it's still fully blocking: lnd won't start up until the cache is populated. However there's no reason for this to be the case, as things like forwarding HTLCs don't actually require access to the cache. Instead, we should modify things such that lnd can start up as soon as the graph is open, and callers block until cache population is complete.