bugfix: Added lock around block writes to reduce memory pressure

Since downloader insert is an asynchronous process, writing thousands of
large blocks into the DB at the same time is very memory intensive.
Adding this lock will make sure that only one block is written at a time
and from the testing in the garden environment this has shown improved
performance in memory use while syncing

core/core.go

@@ -53,6 +53,8 @@ type Core struct {
 	appendQueue     *lru.Cache
 	processingCache *lru.Cache
 
+	writeBlockLock sync.RWMutex
+
 	quit chan struct{} // core quit channel
 }
 
@@ -352,6 +354,9 @@ func (c *Core) Stop() {
 
 // WriteBlock write the block to the bodydb database
 func (c *Core) WriteBlock(block *types.Block) {
+	c.writeBlockLock.Lock()
+	defer c.writeBlockLock.Unlock()
+
 	if c.sl.IsBlockHashABadHash(block.Hash()) {
 		return
 	}