Update style, add comments.

lua:
comments was added.

lua:
box.fiber.status function was added. The function returns current fiber
status, like a cocoroutine.status() function in lua standard library.

core:
fiber_is_caller function was added. The function returns true if
checking fiber is caller of current working fiber.

Flushing stream after write request was added.

Establish connection routine was updated: now socket uses TCP_NODELAY
option.

Clarify comment in index_count() function.

- Add space_n() and index_n() utility functions.
- Add index_is_primary() utility function.
- Rework index initialization and iteration sequences.
- Coding style fixes here and there.

Conflicts:
	test/CMakeLists.txt

Code review fixes for branch tree-index-opt:
to avoid unnecessary copying and settle the issue of
ownership of key_def structures, move their ownership
to struct space.

Now Index uses its key_def by pointer, and all key_defs are
allocated and freed separately in struct space
allocation/deallocation functions.

Fix coding style (use Lisp/Emacs style function
call style, as per LKML which we follow).

validation added.

- use dedicated comparison functions for each Tree type to avoid method
  dispatch within the functions;
- a number of cosmetic changes.

Valgrind tests was fixed.