Remove suggest transport action

[areek]

Currently suggest through _search & _suggest endpoints use different transport actions.
The _suggest endpoint uses its own transport action to avoid executing search & aggregration
phases. This change removes dedicated suggest transport action and adds optimization to
shortcut to suggest phase when _search endpoint is used exclusively for suggestions.

This implies:

GET /_suggest
{....}

is actually sugar for:

GET /_search
{
  "suggest" : ....
}

Besides reducing code and complexity of dealing with multiple execution paths for suggest,
this change has two major benefits:

• simplifies implementing document fetching for completion
• allows using the search context to process suggestions
  • we can use post_filter / query for filtering suggestions in completion
  • completion can respect _type (through contexts)
  • simplify phrase suggester parameters

API Changes:

This change removes suggest action from the client API:

client().prepareSuggest().addSuggestion("foo", ...)

is equivalent to:

client().prepareSearch().suggest(new SuggestBuilder().addSuggestion("foo", ...))

Behaviour changes:
now we use the search threadpool instead of suggest for suggest-only requests
(would appreciate some feedback there) and the merged suggest stats are representative
of all suggest-only requests either from _suggest or _search endpoints

relates to #10217
simplifies #13576

[s1monw]

+100 for the diff stats :)

[s1monw]

@areek this only breaks the java API right?

[areek]

@s1monw yes, and the suggest enum for the search stats request is a no-op as the suggest stats are merged with the search stats.

[s1monw]

@s1monw yes, and the suggest enum for the search stats request is a no-op as the suggest stats are merged with the search stats.

I was more curious about the REST endpoints but I guess I will see in a second when I am reviewing :)

[s1monw]

can we name this isSuggestOnly()?

[s1monw]

I left some minor comments - this looks fantastic! Please add the stats change to the breaking docs too.

thanks so much for this cleanup

[areek]

Thanks @s1monw for the review! I added breaking docs for the java api and merging suggest stats into search stats.
we don't use the suggest threadpool at all now (was used only by the dedicated suggest transport action). any thoughts? if we are ok with using search threadpool, we should remove the suggest threadpool all together.

[s1monw]

+1 on removing suggest threadpool

[s1monw]

LGTM - awesome lets remove the threadpool in a sep PR, we also need a breaking doc addition for that

[areek]

Thanks @s1monw for the review! I will open a PR for removing suggest threadpool