WIP: optionally configure DirectDruidClient to use /druid/v3 instead of /druid/v2

[himanshug]

for talking to historicals and accumulate the response context.

DEPENDS ON #3319 and is reviewable after same is merged , creating PR for lookahead

enabled with "druid.broker.httpClient.useV3QueryUrl=true" in runtime.properties.

response context from broker would include response context from various historicals as...

{
"result": [.... ]
"context": {
.....
"historical1" : {...},
"historical2" : {....}
}

[drcrallen]

Proposing as part of 0.9.3 for now as not to block 0.9.2

[drcrallen]

I propose using a factory here with the two default implementations being a queryv2 and a queryv3 factory which can generate the proper return value.

[fjy]

@himanshug is this still WIP?

[himanshug]

@fjy it was ready but depends on #3319 so removing from 0.9.3 till that is merged. also i'm planning to make additional changes to #3319 which will require changes here too. see #3319 (comment)

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