Implement rtti-like scheme to export C++ type information to FFI.

Remove iproto_connection_count(), it's not possible to maintain
this counter in multi-threaded environment.

Conflicts:
	src/box/iproto.cc
	src/box/lua/call.cc

In case output is multiplexed, obuf_rollback_to_svp could
roll back to a very early  in the output buffer, since
svp is taken before a possible yield.

This is a regression introduced in the patch which
removes iproto_port from Lua calls.

Test case

*)added proper cleanup
*)minor fixes and checks

vclock_get() can return -1 in this case.
Probably introduced by 311551f9

Push a workaround for infinite loop in backtarce_foreach() when
fiber == self: do not try to print trace of the current fiber.

Fix a subtle bug with blocked event loop when joining checkpoint thread.
Celeanup included headers in memtx_engine.cc

Implement a cap on the size of the worker fiber pool in iproto (1024
fibers by default).
Don't maintain too many fibers in the fiber cache of iproto.

Correct the assertion to match the logic of the code.

Conflicts:
	doc/sphinx/book/box/box_introspection.rst

Chagne iobuf to use slab_cache directly, bypassing an intermediate
allocation layer - this is necessry to make iobuf multi-threading
safe.

Introduce mpstream API to lua-msgpack encoder, which allows it
to encode msgpack into any memory source. The streaming API
should make it possible to encode even directly to network.

Patch region allocator to support the new API.
Patch iobuf/obuf output buffer of iproto to support the new API.

Extract Exception::clear(), Exception::move(), fiber()->exception
into struct diag methods. This refactoring makes desired logic
to be more clear.