lua: add fiber.top() listing fiber cpu consumption

Implement a new function in Lua fiber library: top(). It returns a table
containing fiber cpu usage stats. The table has two entries:
"cpu_misses" and "cpu". "cpu" itself is a table listing all the alive
fibers and their cpu consumtion.
The patch relies on CPU timestamp counter to measure each fiber's time
share.

Closes #2694

@TarantoolBot document
Title: fiber: new function `fiber.top()`

`fiber.top()` returns a table of all alive fibers and lists their cpu
consumption. Let's take a look at the example:
```
tarantool> fiber.top()
---
- cpu:
    107/lua:
      instant: 30.967324490456
      time: 0.351821993
      average: 25.582738345233
    104/lua:
      instant: 9.6473633128437
      time: 0.110869897
      average: 7.9693406131877
    101/on_shutdown:
      instant: 0
      time: 0
      average: 0
    103/lua:
      instant: 9.8026528631511
      time: 0.112641118
      average: 18.138387232255
    106/lua:
      instant: 20.071174377224
      time: 0.226901357
      average: 17.077908441831
    102/interactive:
      instant: 0
      time: 9.6858e-05
      average: 0
    105/lua:
      instant: 9.2461986412164
      time: 0.10657528
      average: 7.7068458630827
    1/sched:
      instant: 20.265286315108
      time: 0.237095335
      average: 23.141537169257
  cpu_misses: 0
...

```
The two entries in a table returned by `fiber.top()` are
`cpu_misses` and `cpu`.

`cpu` itself is a table whose keys are strings containing fiber ids and
names.
The three metrics available for each fiber are:
1) instant (per cent),
which indicates the share of time fiber was executing during the
previous event loop iteration
2) average (per cent), which is calculated as an exponential moving
average of `instant` values over all previous event loop iterations.
3) time (seconds), which estimates how much cpu time each fiber spent
processing during its lifetime.

More info on `cpu_misses` field returned by `fiber.top()`:
`cpu_misses` indicates the amount of times tx thread detected it was
rescheduled on a different cpu core during the last event loop
iteration.
fiber.top() uses cpu timestamp counter to measure each fiber's execution
time. However, each cpu core may have its own counter value (you can
only rely on counter deltas if both measurements were taken on the same
core, otherwise the delta may even get negative).
When tx thread is rescheduled to a different cpu core, tarantool just
assumes cpu delta was zero for the latest measurement. This loweres
precision of our computations, so the bigger `cpu misses` value the
lower the precision of fiber.top() results.

Fiber.top() doesn't work on arm architecture at the moment.

Please note, that enabling fiber.top() slows down fiber switching by
about 15 per cent, so it is disabled by default.
To enable it you need to issue `fiber.top_enable()`.
You can disable it back after you finished debugging  using
`fiber.top_disable()`.
"Time" entry is also added to each fibers output in fiber.info()
(it duplicates "time" entry from fiber.top().cpu per fiber).
Note, that "time" is only counted while fiber.top is enabled.