Refactor search with updateByQuery

[f-galland]

Description

This PR replaces the SearchThread class with a new one that uses updateByQuery to conform to the latest command-manager design requirements.

Issues

Closes #249

[f-galland]

It looks like this works ok even for larger numbers of commands. I tested this with the following script:

#!/bin/bash

command='{"source":"Engine","user":"user53","target":{"id":"target4","type":"agent"},"action":{"name":"restart","args":{"arg1":"/path/to/executable/arg6"},"version":"v4"},"timeout":30}'
commands='{"commands": ['

for i in {1..744}
do
  commands+="$command"
  commands+=","
done

commands=${commands::-1}
commands+=']}'

echo $commands > commands.json

curl http://localhost:9200/_plugins/_command_manager/commands -H 'Content-Type: application/json' -d "$(cat commands.json)"

The number of commands, 744, was chosen as it was the maximum curl would allow me to send at once. I fired this repeatedly and then waited for the timeouts.

I then queried the engine for commands with failure status:

#!/bin/bash


echo -ne 'Failed:\n\n'
curl http://localhost:9200/.commands/_count -H 'Content-Type: application/json' -d '
{
  "query": { 
    "bool": { 
      "filter": [ 
        { "term":  { "command.status": "failure" }}
      ]
    }
  }
}
'
echo -ne '\n\nPending:\n\n'
curl http://localhost:9200/.commands/_count -H 'Content-Type: application/json' -d '
{
  "query": { 
    "bool": { 
      "filter": [ 
        { "term":  { "command.status": "pending" }}
      ]
    }
  }
}
'

This is what I got:

Failed:

{"count":3720,"_shards":{"total":1,"successful":1,"skipped":0,"failed":0}}

Pending:

{"count":0,"_shards":{"total":1,"successful":1,"skipped":0,"failed":0}}