Reduce the capacity of countPersistedHeadChunks.

The capacity is basically how many persisted head chunks we will count
at most while doing other things, in particular checkpointing. To
limit the amount of already counted head chunks, keep this number low,
otherwise we will easily checkpoint too often if checkpoints take long
anyway.

storage/local/storage.go

@@ -154,7 +154,7 @@ func NewMemorySeriesStorage(o *MemorySeriesStorageOptions) (Storage, error) {
 		persistStopped:  make(chan struct{}),
 		persistence:     p,
 
-		countPersistedHeadChunks: make(chan struct{}, 1024),
+		countPersistedHeadChunks: make(chan struct{}, 100),
 
 		evictList:     list.New(),
 		evictRequests: make(chan evictRequest, evictRequestsCap),