Update V3__queue_add_priority.sql

[andyyumiao]

Remove SQL grammar when adding column : 'IF NOT EXISTS' , it's for PostgreSQL.

[codecov]

Codecov Report

Merging #1616 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master    #1616     +/-   ##
===========================================
- Coverage     64.16%   64.06%   -0.1%     
  Complexity     3415     3415             
===========================================
  Files           268      268             
  Lines         16154    16154             
  Branches       1446     1446             
===========================================
- Hits          10365    10349     -16     
- Misses         5071     5087     +16     
  Partials        718      718

Impacted Files	Coverage Δ	Complexity Δ
...ductor/dao/cassandra/CassandraEventHandlerDAO.java	68.49% <0%> (-10.96%)	15% <0%> (ø)
.../conductor/dao/cassandra/CassandraMetadataDAO.java	54.73% <0%> (-4.22%)	24% <0%> (ø)

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[coveralls]

Pull Request Test Coverage Report for Build 3890

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 16 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 68.922%

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Files with Coverage Reduction	New Missed Lines	%
cassandra-persistence/src/main/java/com/netflix/conductor/dao/cassandra/CassandraEventHandlerDAO.java	8	69.86%
cassandra-persistence/src/main/java/com/netflix/conductor/dao/cassandra/CassandraMetadataDAO.java	8	55.79%

Totals
Change from base Build 3866:	-0.1%
Covered Lines:	11348
Relevant Lines:	16465

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💛 - Coveralls

[apanicker-nflx]

Please resolve conflicts.