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Totktonada 
Add merger for tuple streams

Fixes #3276.

@TarantoolBot document
Title: Merger for tuple streams

## API and basic usage

The following example demonstrates API of the module:

```lua
local msgpack = require('msgpack')
local net_box = require('net.box')
local buffer = require('buffer')
local merger = require('merger')

-- The format of key_parts parameter is the same as
-- `{box,conn}.space.<...>.index.<...>.parts` (where conn is
-- net.box connection).
local key_parts = {
    {
        fieldno = <number>,
        type = <string>,
        [ is_nullable = <boolean>, ]
        [ collation_id = <number>, ]
        [ collation = <string>, ]
    },
    ...
}

-- Create the merger context.
local ctx = merger.context.new(key_parts)

-- Optional parameters.
local opts = {
    -- Output buffer, only for merger.select(ctx, <...>).
    [ buffer = <buffer>, ]
    -- Ascending (default) or descending result order.
    [ descending = <boolean>, ]
    -- Optional callback to fetch more data.
    [ fetch_source = <function>, ]
}

-- Prepare buffer source.
local conn = net_box.connect('localhost:3301')
local buf = buffer.ibuf()
conn.space.s:select(nil, {buffer = buf}) -- read to buffer
buf.rpos = assert(net_box.check_iproto_data(buf.rpos,
    buf.wpos - buf.rpos))

-- We have three sources here.
local sources = {
    buf,                   -- buffer source
    box.space.s:select(),  -- table source
    {box.space.s:pairs()}, -- iterator source
}

-- Read the whole result at once.
local res = merger.select(ctx, sources, opts)

-- Read the result tuple per tuple.
local res = {}
for _, tuple in merger.pairs(ctx, sources, opts) do
    -- Some stop merge condition.
    if tuple[1] > MAX_VALUE then break end
    table.insert(res, tuple)
end

-- The same in the functional style.
local function cond(tuple)
    return tuple[1] <= MAX_VALUE
end
local res = merger.pairs(ctx, sources, opts):take(cond):totable()
```

The basic case of using merger is when there are M storages an... (continued)

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Jobs
ID Job ID Ran Files Coverage
4 15072.4 (TARGET=coverage) 365
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 Travis Job 15072.4
Source Files on build 15072