Avoid creating subworkflow object every time

[jun-he]

Avoid creating subworkflow object every time when handling subworkflow tasks.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (dev@185e436). Click here to learn what that means.
The diff coverage is 100.00%.

@@          Coverage Diff           @@
##             dev    #1954   +/-   ##
======================================
  Coverage       ?   65.60%           
  Complexity     ?     3935           
======================================
  Files          ?      301           
  Lines          ?    18525           
  Branches       ?     1685           
======================================
  Hits           ?    12153           
  Misses         ?     5533           
  Partials       ?      839           

Impacted Files	Coverage Δ	Complexity Δ
...lix/conductor/core/execution/WorkflowExecutor.java	80.19% <100.00%> (ø)	182.00 <3.00> (?)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 185e436...63fc840. Read the comment docs.

[coveralls]

Pull Request Test Coverage Report for Build 4723

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.004%) to 70.078%

---

Files with Coverage Reduction	New Missed Lines	%
cassandra-persistence/src/main/java/com/netflix/conductor/dao/cassandra/CassandraMetadataDAO.java	2	55.44%

Totals
Change from base Build 4721:	0.004%
Covered Lines:	12982
Relevant Lines:	18525

---

💛 - Coveralls