Fix PostingsFormat in KNN Codec (#236)

[vamshin]

• postings list fix

Issue #, if available:
#227

Description of changes:
#236

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[codecov]

Codecov Report

Merging #248 into opendistro-1.10 will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@                  Coverage Diff                  @@
##             opendistro-1.10     #248      +/-   ##
=====================================================
- Coverage              77.87%   77.76%   -0.11%     
- Complexity               308      311       +3     
=====================================================
  Files                     50       50              
  Lines                   1184     1192       +8     
  Branches                 100      100              
=====================================================
+ Hits                     922      927       +5     
- Misses                   218      221       +3     
  Partials                  44       44              

Impacted Files	Coverage Δ	Complexity Δ
...csearch/knn/index/codec/KNN86Codec/KNN86Codec.java	92.00% <100.00%> (+2.00%)	14.00 <4.00> (+2.00)
...roforelasticsearch/knn/plugin/KNNCodecService.java	75.00% <100.00%> (+8.33%)	3.00 <1.00> (+1.00)
...oforelasticsearch/knn/plugin/KNNEngineFactory.java	100.00% <100.00%> (ø)	3.00 <2.00> (ø)
...istroforelasticsearch/knn/index/KNNIndexCache.java	92.30% <0.00%> (-2.89%)	35.00% <0.00%> (ø%)

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[vamshin]

Manually verified suggest Field to check PostingsFormat fix

curl -X PUT "localhost:9200/music?pretty" -H 'Content-Type: application/json' -d'

{
"settings": {
"index": {
"knn": true
}
},
"mappings": {
"properties": {
"suggest": {
"type": "completion"
}
}
}
}
'

curl -X POST "localhost:9200/music/_doc/1?refresh=true" -H 'Content-Type: application/json' -d'
{
"suggest" : {
"input": [ "Nevermind", "Nirvana" ],
"weight" : 34
}
}
'

curl -X POST "localhost:9200/music/_search?pretty" -H 'Content-Type: application/json' -d'
{
"suggest": {
"song-suggest": {
"prefix": "nir",
"completion": {
"field": "suggest"
}
}
}
}
'

Output:-

{
"took" : 92,
"timed_out" : false,
"_shards" : {
"total" : 1,
"successful" : 1,
"skipped" : 0,
"failed" : 0
},
"hits" : {
"total" : {
"value" : 0,
"relation" : "eq"
},
"max_score" : null,
"hits" : [ ]
},
"suggest" : {
"song-suggest" : [
{
"text" : "nir",
"offset" : 0,
"length" : 3,
"options" : [
{
"text" : "Nirvana",
"_index" : "music",
"_type" : "_doc",
"_id" : "1",
"_score" : 34.0,
"_source" : {
"suggest" : {
"input" : [
"Nevermind",
"Nirvana"
],
"weight" : 34
}
}
}
]
}
]
}
}