Error on setting callable table as trigger

Tarantool version: 1.10.3-6-g5dc6d79

OS version:

$ lsb_release  -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.5 LTS
Release:	16.04
Codename:	xenial

Bug description: It is possible to set function as on_replace_trigger, however using callable table is impossible (even though lua code in tarantool seem to allow it).It would be nice if a trigger could hold it's description inside. It would be possible with a table, but not possible with function (without any strange code inside the trigger)

Steps to reproduce:

box.cfg{}

box.schema.create_space('t')
box.space.t:format({ { name = 'id', type = 'number' } })
box.space.t:create_index('primary', { type = 'tree', parts = { 'id' } })

NUMBER = 0

local NUMBER_INC_TABLE = setmetatable({}, { __call = function(self, old, new) NUMBER = NUMBER + 1 end})
local NUMBER_INC_FUNC  = function(old, new) NUMBER = NUMBER + 1 end

local ok, err = pcall(function() box.space.t:on_replace(NUMBER_INC_FUNC) end)
if not ok then
	error(("%s ERROR on setting function as on_replace_trigger: %s"):format(_TARANTOOL, err))
end

local ok, err = pcall(function() box.space.t:on_replace(NUMBER_INC_TABLE) end)
if not ok then
	error(("%s ERROR on setting callable table as on_replace_trigger: %s"):format(_TARANTOOL, err))
end

Start by running the provided Lua reproduction on Tarantool 1.10.3-6-g5dc6d79 and trace the on_replace trigger handling path for callable tables. The fix is complete when the callable table can be registered like the function and the reproduction no longer reports an error.