Implement blueprint 'add-delete-fields':
https://blueprints.launchpad.net/tarantool/+spec/add-and-delete-fields-in-tuple

Make it possible to add new fields with UPDATE, and
delete existing fields.

Rewrite the UPDATE algorithm to pre-compute new tuple
length and do most of the work in place, without
allocating temporary buffer for fields.

Add Connector/C support for new UPDATE options.

Add and update tests.

Joint effort by Konstantin Shulgin (initial implementation),
Dmitry Simonenko (Connector/C) and
Konstantin Osipov (spec/code review).

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.