Move towards a uniform include scheme where header files are
stored along with the sources files, in the same directory.

We had to begin to use this scheme thanks to use of subprojects,
and a mix of two approaches turns the code base into a mess.

Move recovery to box/. Now only replication.cc and replica.cc
depend on recovery.

Remove unused headers from a bunch of files. This should reduce
recompile time and ease future refactoring.

slab_arena allows to have multiple allocation
sources of differnt type for different purposes.
E.g. one can have MAP_PRIVATE arena for runtime
memory and MAP_SHARED for tuples.

The arena can be shared across multiple threads.
Implement such arena, cover with tests, use
it for region allocators.

Conflicts:
	CMakeLists.txt

Conflicts:
	src/CMakeLists.txt
	src/box/box_lua.cc
	src/lua/session.cc

The patch should use struct session, which is non-existent
in 1.5, so at least make session_storage_destroy a bit faster.

Conflicts:
	include/tarantool.h
	src/box/box.cc
	src/replica.cc
	src/replication.cc

Conflicts:
	src/admin.cc
	src/admin.rl
	src/box/bitset_index.cc
	src/box/hash_index.cc
	src/box/hash_index.h
	src/box/space.cc
	src/box/space.h
	test/box/admin.result

This patch implements show index and shows memory waste of the
slab allocator.

Conflicts:
	doc/user/configuration-reference.xml
	doc/user/persistence-architecture.xml
	doc/user/stored-procedures.xml

Conflicts:
	test/lib/tarantool_server.py
	test/lib/test_suite.py
	test/lib/unittest_server.py