new handles for fiber parent backtrace feature

Related dev. issue(s): https://github.com/tarantool/tarantool/commit/3116c4e7d35e75a70a6b4b6d8e9664f2f7c5de8d.

Product: Tarantool
Since: 2.10.0-rc1
Root document:
https://www.tarantool.io/ru/doc/latest/dev_guide/reference_capi/fiber/
https://www.tarantool.io/ru/doc/latest/reference/reference_lua/fiber/
SME: @ CuriousGeorgiy

Details

With backtrace implementation reworked in afc100d, we can now
efficiently collect unwinding information about parents at fiber
creation time:

• Add core_backtrace_cat helper for concatenating backtraces.
• Add 'parent_bt' field to struct fiber for storing fiber parent's
  backtrace.
• Add fiber_parent_backtrace_enabled and corresponding C/Lua toggles for
  controlling parent backtrace collection for newly created fibers.

local fiber = require('fiber')
local log = require('log')
local yaml = require('yaml')

local function foo()
log.info("%s", yaml.encode(fiber.info()[fiber.self()[id]].backtrace))
end

-- Parent backtrace collection feature is disable by default.
fiber.parent_backtrace_enable()
-- Backtrace will contain also parent's backtrace.
fiber.create(foo)

fiber.parent_backtrace_disable()
-- Backtrace will not contain parent's backtrace.
fiber.create(foo)

local function bar()
-- Fibers created before enabling parent backtrace feature will not be
-- contained in backtrace.
fiber.parent_backtrace_enable()
fiber.create(foo)
end

-- Grandchild will not contain child's backtrace.
fiber.create(bar)

Definition of done

• You have agreed with @CuriousGeorgiy which sections should have information added and which parameters should be updated
• The necessary sections have been updated.

Start with the linked C API and Lua fiber reference pages and review their existing backtrace and fiber creation sections. Confirm with @CuriousGeorgiy which sections need the parent backtrace feature and which parameters require updates. Done means the agreed sections and parameters document the new handles and toggles accurately.