add sync function for incremental update

[fishjam]

Requirements

• current esm use scroll + bulk to migrate data between es index
• can NOT support delete dest records when the records already delete in source index unless use force to delete the dest index first
• it will bulk all source records to dest index even though the dest records hasn't changed ( can confirmed by GET //_stats?pretty index_total count )

Solutions

• add --sync to scroll and compare the source and dest index records, and just index/update/delete the changed records
• example:

  esm --sync --count=10000 --sleep=1
   --source=http://localhost:9200 --src_indexes=bank 
   --dest=http://localhost:9200 --dest_index=bank_sync

Implementation

• Correctness and memory consumption need to be considered
• Scroll order by _id => compare records => index/update/delete for changed records => nextscroll for src/dst

Other changes

• add config: SortField, TruncateOutFile, SkipFields, so can dump es index to local file and compare the data easily

   esm --sort=_id --source=http://localhost:9200 --src_indexes=bank --truncate_output --skip=_index --output_file=src.json
   esm --sort=_id --source=http://localhost:9200 --src_indexes=bank_sync --truncate_output --skip=_index --output_file=dst.json
   diff -W 200 -ry --suppress-common-lines src.json dst.json

• add some functions in esapi, ClusterVersion() and DeleteScroll(), add ParseEsApi
• move bulk.go to migrator.go, and add some functions
• refactor all http method(GET/Post/DoRequest) to sinle Request method, and support proxy. BTW: seems this change also fix some erro: example: in original version, try use --regenerate_id --repeat_times=10 to generate some test data, but it can not work(can not generate target index).
• delete some commented and useless code
• run following command to format code: gofmt -l -w .

TODO

• 1. now use single goroutine to scroll for src + dest index, and insert/update/delete records, need consider performance after more check
• 2. add range condition support( example: modifydate gt xxx ), so can do sync more effective

[fishjam]

@medcl I found you have merged it, and could you releasae new version ? thank you.

[medcl]

Released, Thanks