add conf entry mysql.engine to change storage engine of mysql, defaul…

[wenzhenghu]

add conf entry mysql.engine to change storage engine of mysql
default innodb, could change to rocksdb, but not myisam, as hugegraph will create large index that myisam could not support ("Specified key was too long; max key length is 1000 bytes")

[codecov]

Codecov Report

Merging #553 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master    #553      +/-   ##
===========================================
+ Coverage     70.19%   70.2%   +<.01%     
  Complexity     3418    3418              
===========================================
  Files           216     216              
  Lines         16461   16470       +9     
  Branches       2351    2350       -1     
===========================================
+ Hits          11555   11562       +7     
- Misses         3650    3654       +4     
+ Partials       1256    1254       -2

Impacted Files	Coverage Δ	Complexity Δ
...du/hugegraph/backend/store/mysql/MysqlOptions.java	95.65% <100%> (+0.41%)	3 <0> (ø)	⬇️
...aidu/hugegraph/backend/store/mysql/MysqlStore.java	73.25% <100%> (-0.16%)	34 <0> (ø)
...aidu/hugegraph/backend/store/mysql/MysqlTable.java	77.37% <100%> (+0.6%)	71 <2> (+1)	⬆️
...ain/java/com/baidu/hugegraph/task/TaskManager.java	60.86% <0%> (-4.35%)	10% <0%> (-1%)
...n/java/com/baidu/hugegraph/task/TaskScheduler.java	51.45% <0%> (-1.25%)	26% <0%> (-1%)
...du/hugegraph/backend/tx/GraphIndexTransaction.java	80% <0%> (+0.47%)	146% <0%> (ø)	⬇️
...u/hugegraph/backend/store/mysql/MysqlSessions.java	70.73% <0%> (+0.6%)	18% <0%> (+1%)	⬆️

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[javeme]

reopened from https://github.com/hugegraph/hugegraph/pull/555