Support PostgreSQL and CockroachDB backends

[zhoney]

implemented: #441

Change-Id: I979affd9f8b2916e6696672fc6e3fda75f9b76da

[codecov]

Codecov Report

Merging #484 into master will increase coverage by 0.38%.
The diff coverage is 85.89%.

@@             Coverage Diff              @@
##             master     #484      +/-   ##
============================================
+ Coverage     68.99%   69.38%   +0.38%     
- Complexity     3280     3336      +56     
============================================
  Files           208      215       +7     
  Lines         16193    16454     +261     
  Branches       2334     2354      +20     
============================================
+ Hits          11172    11416     +244     
- Misses         3774     3778       +4     
- Partials       1247     1260      +13

Impacted Files	Coverage Δ	Complexity Δ
...aidu/hugegraph/backend/store/mysql/MysqlStore.java	73.41% <0%> (+1.89%)	34 <0> (+3)	⬆️
...u/hugegraph/backend/store/mysql/MysqlSessions.java	69.75% <0%> (+0.8%)	16 <0> (+1)	⬆️
...egraph/backend/store/cassandra/CassandraTable.java	80.07% <0%> (+1.21%)	73 <0> (-2)	⬇️
...du/hugegraph/backend/store/mysql/MysqlOptions.java	95.23% <100%> (+1.12%)	3 <0> (+1)	⬆️
...hugegraph/backend/store/mysql/MysqlSerializer.java	82.75% <100%> (+0.3%)	20 <1> (+1)	⬆️
...raph/backend/store/postgresql/PostgresqlStore.java	100% <100%> (ø)	2 <2> (?)
...idu/hugegraph/backend/store/mysql/MysqlTables.java	86.86% <100%> (+1.23%)	1 <1> (+1)	⬆️
...h/backend/store/postgresql/PostgresqlSessions.java	61.11% <61.11%> (ø)	3 <3> (?)
...com/baidu/hugegraph/backend/store/TableDefine.java	57.14% <63.63%> (-19.33%)	8 <1> (+2)
.../hugegraph/backend/serializer/TableSerializer.java	81.34% <66.66%> (-1.45%)	52 <9> (+5)
... and 18 more

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