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)

Conflicts:
	src/box/alter.cc
	test/app/snapshot.test.lua

* style fixes
* fix a bug in key opts decoder (alter.cc)
* ensure dimension is visible in box.space
* rebuild bootstrap.snap to use the new format

cmake 2.8.9 doesn't have CMAKE_CXX_COMPILER_VERSION

- fix net.box to satisfy strict.lua requirements
- after review fixes
- fix buffer:checksize() in buffer.lua

When entire contents of the output buffer is flushed, we must reset
write start and end positions. If the buffer can be recyced, they
both are reset to point to the beginning of the buffer, otherwise write start
is advanced to match write end.

The old code would always execute *begin = *end assignment, regardless
of whether it's case 1) (full reset) or 2) (advancing write start).

gcc 4.8 could reorder this assignment, and put it in front of conditional
iobuf_reset(). In that case, write start could point beyond write end,
since iobuf_reset() resets write end to zero.

A funny bug which would hit only under heavy load, only if there are
-O2 optimizations, and no debug prints.

Make box.slab.stats() output CALL compatible.

Reflect in slab stats the quota size and index memory statistics.

arena_used_ratio - how much of arena is used
items_used_ratio - how much memory is actualy used for items,
given that there is some fragmentation. Doesn't reflect index pages
arena_used - how much of the quota is used (i.e. how much memory
is used for index and tuple data)
arena_size  - this is the same as box.cfg.slab_alloc_arena.
quota_size - how much of the quota is defined (box.cfg.slab_alloc_arena)
quota_used - how much of the quota is used (reflects the memory taken
by all slabs, including slabs in caches)

Incompatible change:
Extract detailed slab statistics to a separate call, box.slab.stats()

Fixes gh-849 'Compile error on CentOS'

Expliclitly prohibit Gcc versions which do not support lambdas.

Do stuff in box_free() only if initialization has succeeded.

Extract index_find() logic into space.h

- upsert semantics is update or insert, not "always update"
- upsert doesn't return tuple
- bind upsert as tuple method in Lua (useful for QA)
- ensure upsert is properly encoded in xlog