tuple: pass global ibuf explicitly where possible

Code in lua/tuple.c used global tarantool_lua_ibuf in many places
relying on it never being changed and not reused by other code
until a yield.

But it is not so. In fact, as it was discovered in #5632, in any
Lua function may be started GC. Any GC handler might touch some
API also using tarantool_lua_ibuf inside.

This makes the first usage in lua/tuple.c invalid - the buffer
could be reset or reallocated or its wpos/rpos could change during
GC.

In order to fix this, first of all there should be clear points
where the buffer is taken, and where it becomes not needed
anymore.

The patch makes code in lua/tuple.c take tarantool_lua_ibuf when
it is needed first time. Not during usage. The same is done for
the fiber region for the API symmetry.

Part of #5632