Conflicts:
	src/CMakeLists.txt
	src/ffisyms.cc
	src/lua/init.cc
	test/unit/CMakeLists.txt

This patch adds four new functions to API:

  * luaL_checkuint64()
  * luaL_touint64()
  * luaL_checkint64()
  * luaL_toint64()

This functions are needed to rewrite request encoding in net.box.

The origin was that 'vmdef.lua' was generated in
'${CMAKE_BINARY_DIR}/third_party/luajit/jit/', but was searched in
'${CMAKE_SOURCE_DIR}/...'.
ALSO: when we start to generate *.c files from luajit's libraries we made
statement, that we don't built with OS pre-built luajits.

sio_move_iov is not built to advance both obuf_svp and iov,
tweak the code to make sure it does what's needed.

gh-983 still needs a test case.

In case of partial write the savepoint was advanced beyond end position.

With structured configuration, i.e. a Lua table for a list of
replication peers, it is difficult to pass a configuration option
to the hook function as a single C object. Don't pass
options in, but query them from the hook function with cfg_get*
API.

As a result, all "change" hooks have the same signature now,
a step closer to the pluggable configuration API.

Introduce a layer of wrappers for a number of internal box functions,
dealing with accss to spaces and indexes.

These wrappers:
    * don't throw exceptions
    * have a common prefix box_
    * are exported in the server development headers

Rewrite Lua C bindings to use the public API described above.
Rewrite Lua FFI bindings to do the same.
Add test.

- update comments
- fix the logic of partial write recovery (still needs a test)