Use "merger" for fetch&sort tuples on router

[olegrok]

Before this patch we used hand-crafted heap to sort tuples on
router. This patch removes it and changes to "merger" module
written in C.

"Merger" is available since Tarantool 2.2. Later "tuple-merger"
was introduced it's a version that is compatible with some
Tarantool 1.10 versions. In fact support several Tarantool
versions is quite complex task. Some msgpack helpers is not
avaliable in Tarantool 1.10 to solve this problem small
compatibility lawer is implemented.
Also "tuple-keydef" doesn't support collation_id options and it's
not so for built-in "key_def" module. We need to normalize index
parts and change "collation_id" to "collation" when create
"key_def" instance.

For versions where merger is not supported/available yet
(before 1.10.8 and before 2.3.3) we use previous approach with
lua-implemented heap.

Closes #33

[olegrok]

https://github.com/tarantool/crud/tree/perf-test

Before patch:

SELECT
Fibers count - 200
Connection count - 10
Timeout  - 30.000000
Requests - 102824
Errors - 0
RPS - 3427.466667

After patch:

SELECT
Fibers count - 200
Connection count - 10
Timeout  - 30.000000
Requests - 168745
Errors - 0
RPS - 5624.833333
Ok

[olegrok]

it's better to use ?table|tuple instead of *|cdata

[olegrok]

The same for all places where tuple expected.

[dokshina]

Great patch!