DefaultHandle pop() {
            int size = this.size;
            if (size == 0) {
                if (!scavenge()) {
                    return null;
                }
                size = this.size;
            }
            size--;
            DefaultHandle ret = elements[size];
            if (ret.lastRecycledId != ret.recycleId) {
                throw new IllegalStateException("recycled multiple times");
            }
            ret.recycleId = 0;
            ret.lastRecycledId = 0;
            this.size = size;
            return ret;
        }
Light-weight object pool based on a thread-local stack.
<p/>
Forked from <a href="https://github.com/netty/netty">Netty</a>.

@param <T> the type of the pooled object
jraft-core/src/main/java/com/alipay/sofa/jraft/util/Recyclers.java