Split big batches in ES output plugin

[goshansmails]

Solves #707

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 32 lines in your changes missing coverage. Please review.

Project coverage is 38.09%. Comparing base (e02c4a6) to head (55cdf0f).
Report is 30 commits behind head on master.

Files with missing lines	Patch %	Lines
e2e/file_es/file_es.go	70.96%	18 Missing and 9 partials ⚠️
plugin/output/elasticsearch/elasticsearch.go	85.29%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #708       +/-   ##
===========================================
- Coverage   72.28%   38.09%   -34.19%     
===========================================
  Files         137      140        +3     
  Lines       13985    15033     +1048     
===========================================
- Hits        10109     5727     -4382     
- Misses       3311     8854     +5543     
+ Partials      565      452      -113     

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