refactor: move graph related DB code to graph/db from channeldb

This is a pure refactor commit. It moves over all the graph related CRUD
code from `channeldb` to `graph/db`.

graph/db/reject_cache.go

@@ -0,0 +1,95 @@
+package graphdb
+
+// rejectFlags is a compact representation of various metadata stored by the
+// reject cache about a particular channel.
+type rejectFlags uint8
+
+const (
+	// rejectFlagExists is a flag indicating whether the channel exists,
+	// i.e. the channel is open and has a recent channel update. If this
+	// flag is not set, the channel is either a zombie or unknown.
+	rejectFlagExists rejectFlags = 1 << iota
+
+	// rejectFlagZombie is a flag indicating whether the channel is a
+	// zombie, i.e. the channel is open but has no recent channel updates.
+	rejectFlagZombie
+)
+
+// packRejectFlags computes the rejectFlags corresponding to the passed boolean
+// values indicating whether the edge exists or is a zombie.
+func packRejectFlags(exists, isZombie bool) rejectFlags {
+	var flags rejectFlags
+	if exists {
+		flags |= rejectFlagExists
+	}
+	if isZombie {
+		flags |= rejectFlagZombie
+	}
+
+	return flags
+}
+
+// unpack returns the booleans packed into the rejectFlags. The first indicates
+// if the edge exists in our graph, the second indicates if the edge is a
+// zombie.
+func (f rejectFlags) unpack() (bool, bool) {
+	return f&rejectFlagExists == rejectFlagExists,
+		f&rejectFlagZombie == rejectFlagZombie
+}
+
+// rejectCacheEntry caches frequently accessed information about a channel,
+// including the timestamps of its latest edge policies and whether or not the
+// channel exists in the graph.
+type rejectCacheEntry struct {
+	upd1Time int64
+	upd2Time int64
+	flags    rejectFlags
+}
+
+// rejectCache is an in-memory cache used to improve the performance of
+// HasChannelEdge. It caches information about the whether or channel exists, as
+// well as the most recent timestamps for each policy (if they exists).
+type rejectCache struct {
+	n     int
+	edges map[uint64]rejectCacheEntry
+}
+
+// newRejectCache creates a new rejectCache with maximum capacity of n entries.
+func newRejectCache(n int) *rejectCache {
+	return &rejectCache{
+		n:     n,
+		edges: make(map[uint64]rejectCacheEntry, n),
+	}
+}
+
+// get returns the entry from the cache for chanid, if it exists.
+func (c *rejectCache) get(chanid uint64) (rejectCacheEntry, bool) {
+	entry, ok := c.edges[chanid]
+	return entry, ok
+}
+
+// insert adds the entry to the reject cache. If an entry for chanid already
+// exists, it will be replaced with the new entry. If the entry doesn't exists,
+// it will be inserted to the cache, performing a random eviction if the cache
+// is at capacity.
+func (c *rejectCache) insert(chanid uint64, entry rejectCacheEntry) {
+	// If entry exists, replace it.
+	if _, ok := c.edges[chanid]; ok {
+		c.edges[chanid] = entry
+		return
+	}
+
+	// Otherwise, evict an entry at random and insert.
+	if len(c.edges) == c.n {
+		for id := range c.edges {
+			delete(c.edges, id)
+			break
+		}
+	}
+	c.edges[chanid] = entry
+}
+
+// remove deletes an entry for chanid from the cache, if it exists.
+func (c *rejectCache) remove(chanid uint64) {
+	delete(c.edges, chanid)
+}