update cache_type default value

[lxb1111]

No description provided.

[javeme]

expect l2(lowercase, not L2)

[lxb1111]

ok，code has been updated and resubmitted

[javeme]

can also add some performance comparison results?

[lxb1111]

test plan：
通过配置不同的cache配置，然后启动server；准备好不同的数据集通过jmeter工具对server进行50并发的查询（ID查询点、ID查询边）；观察GC变化、请求的延迟情况。

result：
1、10000w数据集情况下点、边查询从请求延迟时间、GC回收时间来看L2级缓存优于L1级缓存
2、10000w数据集情况下点、边查询L1级缓存延迟时间、GC回收时间都是随着缓存设置的递增呈现非线性递增，且变化明显
3、10000w数据集情况下点、边查询L2级缓存延迟时间随着缓存设置的递增没有明显太大变化，但是GC回收时间是整体趋势为增加的情况

[codecov]

Codecov Report

Merging #1681 (1f04cbf) into master (3b623bc) will increase coverage by 0.55%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1681      +/-   ##
============================================
+ Coverage     66.20%   66.75%   +0.55%     
- Complexity     7022     7053      +31     
============================================
  Files           421      420       -1     
  Lines         34682    34668      -14     
  Branches       4804     4805       +1     
============================================
+ Hits          22960    23143     +183     
+ Misses         9379     9187     -192     
+ Partials       2343     2338       -5     

Impacted Files	Coverage Δ
...n/java/com/baidu/hugegraph/config/CoreOptions.java	99.48% <ø> (ø)
...om/baidu/hugegraph/backend/cache/OffheapCache.java	79.88% <100.00%> (+22.05%)	⬆️
...om/baidu/hugegraph/backend/query/QueryResults.java	70.09% <0.00%> (-1.34%)	⬇️
.../com/baidu/hugegraph/type/define/SchemaStatus.java	80.00% <0.00%> (-0.96%)	⬇️
...java/com/baidu/hugegraph/schema/SchemaElement.java	77.14% <0.00%> (-0.33%)	⬇️
...c/main/java/com/baidu/hugegraph/util/LockUtil.java	84.93% <0.00%> (-0.21%)	⬇️
...in/java/com/baidu/hugegraph/StandardHugeGraph.java	68.47% <0.00%> (-0.20%)	⬇️
...du/hugegraph/schema/builder/IndexLabelBuilder.java	86.95% <0.00%> (-0.08%)	⬇️
...m/baidu/hugegraph/backend/tx/GraphTransaction.java	79.85% <0.00%> (-0.05%)	⬇️
...in/java/com/baidu/hugegraph/schema/IndexLabel.java	77.52% <0.00%> (ø)
... and 26 more

---

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[javeme]

there is ci error:

Error:  Tests run: 717, Failures: 0, Errors: 1, Skipped: 79, Time elapsed: 228.605 s <<< FAILURE! - in com.baidu.hugegraph.core.CoreTestSuite
Error:  testAddEdgeWithInvalidSortkey(com.baidu.hugegraph.core.EdgeCoreTest)  Time elapsed: 0.062 s  <<< ERROR!
java.lang.IllegalArgumentException: Can't contains byte '0x00' in string: '2017-5-27'
	at com.baidu.hugegraph.core.EdgeCoreTest.testAddEdgeWithInvalidSortkey(EdgeCoreTest.java:538)