Do not drop on the floor small buffers

Currently if a buffer from the buffer is too small to satisfy its request then we simply drop it and allocate a new one.

This change puts it back in the pool and then allocates a new one.

tsdb/engine/tsm1/pools.go

@@ -13,6 +13,7 @@ func getBuf(size int) *[]byte {
 	}
 	buf := x.(*[]byte)
 	if cap(*buf) < size {
+		bufPool.Put(x)
 		b := make([]byte, size)
 		return &b
 	}