fix existDatabase(), clearBackend() and rollback() for postgresql

[zhoney]

fixed: #530

Change-Id: I5061eb95abf0e0ef3a6a83a74ced291647d9308d

[codecov]

Codecov Report

Merging #531 into master will increase coverage by 0.21%.
The diff coverage is 73.17%.

@@             Coverage Diff              @@
##             master     #531      +/-   ##
============================================
+ Coverage     70.22%   70.44%   +0.21%     
- Complexity     3418     3444      +26     
============================================
  Files           216      216              
  Lines         16465    16487      +22     
  Branches       2351     2352       +1     
============================================
+ Hits          11563    11614      +51     
+ Misses         3646     3629      -17     
+ Partials       1256     1244      -12

Impacted Files	Coverage Δ	Complexity Δ
...n/java/com/baidu/hugegraph/config/CoreOptions.java	98.59% <ø> (ø)	2 <0> (ø)	⬇️
...src/main/java/com/baidu/hugegraph/HugeFactory.java	68% <100%> (+20.17%)	6 <0> (+2)	⬆️
...backend/store/postgresql/PostgresqlSerializer.java	87.5% <100%> (+4.89%)	6 <0> (ø)	⬇️
...u/hugegraph/backend/store/mysql/MysqlSessions.java	70.85% <66.66%> (-0.06%)	20 <4> (+2)
...h/backend/store/postgresql/PostgresqlSessions.java	70.27% <76.47%> (+5.27%)	8 <3> (+3)	⬆️
...a/com/baidu/hugegraph/backend/query/Condition.java	64.55% <0%> (-0.53%)	23% <0%> (ø)
...n/java/com/baidu/hugegraph/task/TaskScheduler.java	52.28% <0%> (-0.42%)	26% <0%> (-1%)
...raph/backend/store/rocksdb/RocksDBStdSessions.java	69.84% <0%> (+0.61%)	24% <0%> (+1%)	⬆️
.../backend/store/cassandra/CassandraSessionPool.java	59.37% <0%> (+1.04%)	13% <0%> (ø)	⬇️
... and 10 more

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