fix fatal error of missing method parseString

[ylwu-amzn]

Issue #, if available:

Description of changes:
When cluster restarted, will restore model from checkpoint, but will throw fatal errors which will restart cluster.

[2020-10-29T19:43:34,676][ERROR][o.e.b.ElasticsearchUncaughtExceptionHandler] [ad5beb168c48658f8e0d1f227f3395cd] fatal error in thread [elasticsearch[ad5beb168c48658f8e0d1f227f3395cd][get][T#4]], exiting
java.lang.NoSuchMethodError: 'com.google.gson.JsonElement com.google.gson.JsonParser.parseString(java.lang.String)'
	at com.amazon.opendistroforelasticsearch.ad.ml.CheckpointDao.lambda$fromEntityModelCheckpoint$12(CheckpointDao.java:469) ~[?:?]
	at java.security.AccessController.doPrivileged(Native Method) ~[?:?]
	at com.amazon.opendistroforelasticsearch.ad.ml.CheckpointDao.fromEntityModelCheckpoint(CheckpointDao.java:467) ~[?:?]
	at com.amazon.opendistroforelasticsearch.ad.ml.CheckpointDao.lambda$restoreModelCheckpoint$13(CheckpointDao.java:501) ~[?:?]

It's caused by ES cluster using old GSON version which doesn't have parseString method. It takes time find out which package introduces the old version of GSON. This PR change to use an old method "parse" as a workaround.

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[codecov]

Codecov Report

Merging #302 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #302      +/-   ##
============================================
- Coverage     72.03%   72.02%   -0.01%     
- Complexity     1967     1968       +1     
============================================
  Files           199      199              
  Lines          9462     9463       +1     
  Branches        844      844              
============================================
  Hits           6816     6816              
- Misses         2233     2236       +3     
+ Partials        413      411       -2     

Flag	Coverage Δ	Complexity Δ
#plugin	71.43% <100.00%> (-0.01%)	1968.00 <1.00> (+1.00)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...pendistroforelasticsearch/ad/ml/CheckpointDao.java	83.76% <100.00%> (+0.08%)	57.00 <1.00> (ø)
...ransport/DeleteAnomalyDetectorTransportAction.java	52.22% <0.00%> (-4.45%)	15.00% <0.00%> (-1.00%)
...asticsearch/ad/cluster/ADClusterEventListener.java	94.00% <0.00%> (+6.00%)	15.00% <0.00%> (+2.00%)

[yizheliu-amazon]

This happens only when cluster restarts?

[ylwu-amzn]

This happens only when cluster restarts?

See this happen when restarts, but I think it will happen when restore entity model from checkpoint.