fix NebulaPool's not serializable & refactor ssl

[Nicole00]

No description provided.

[codecov-commenter]

Codecov Report

Merging #32 (722aae5) into master (6541047) will increase coverage by 0.20%.
The diff coverage is 69.56%.

@@             Coverage Diff              @@
##             master      #32      +/-   ##
============================================
+ Coverage     39.83%   40.04%   +0.20%     
- Complexity      142      144       +2     
============================================
  Files            46       46              
  Lines          1373     1371       -2     
  Branches        128      128              
============================================
+ Hits            547      549       +2     
+ Misses          776      772       -4     
  Partials         50       50              

Impacted Files	Coverage Δ
...connector/nebula/sink/NebulaBatchOutputFormat.java	46.87% <50.00%> (+1.71%)	⬆️
...bula/connection/NebulaGraphConnectionProvider.java	83.33% <76.47%> (+11.90%)	⬆️

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[HarrisChu]

LGTM