hugegraph-server 用restapi查询了多次之后,内存占用就降不下来

### Expected behavior 期望表现
希望内存占用能够在不用的时候下降

### Actual behavior 实际表现
比如刚启动hugegraph-server的时候内存占用3G,使用多次restapi查询之后,内存会到10g,而且不会下降,求问怎么解决

### Steps to reproduce the problem 复现步骤
1. {step 1}
2. {step 2}
3. {step 3}

### Status of loaded data 数据状态

#### Vertex/Edge summary 数据量
- loaded vertices amount: {like 10 million}
- loaded edges amount: {like 20 million}
- loaded time: {like 200s}

#### Vertex/Edge example 数据示例
{type something here...}

#### Schema(VertexLabel, EdgeLabel, IndexLabel) 元数据结构
{type something here...}

### Specifications of environment 环境信息
- hugegraph version: {like v0.7.4}
- operating system: {like centos 7.4, 32 CPUs, 64G RAM}
- hugegraph backend: {like cassandra 3.10, cluster with 20 nodes, 3 x 1TB HDD disk each node}