graph/db: async graph cache population

[ellemouton]

Let the channel graph cache be populated asynchronously on startup. While the cache is being populated, the graph is still available for queries, but all read queries will be served from the database until the cache is fully populated. This new behaviour can be opted out of via the new --db.sync-graph-cache-load option.

Fixes #6187 Replaces #8919 Depends on https://github.com/lightningnetwork/lnd/pull/10068

[ellemouton]

/gemini review

[djkazic]

If there are writes to the underlying graph during cache population are these queued to be written into the cache, or will these entries just experience a cache miss when a read happens later?

[lightninglabs-deploy]

@ellemouton, remember to re-request review from reviewers when ready