Refactoring: remove the hack with storing txn in fiber.

Continue cherry-picking refactoring ideas from txn-refine
tree.
We only needed to store txn in fiber because in port_lua
implementation we needed to access lua_State.
Instead of keeping this indirect dependency, pass around ports
explicitly.
On the same token, make ports Objective C classes:
this is necessary to make them stateful.
In port_lua state keep a pointer to lua thread, and allocate
a new port_lua instance on each lbox_process call.
The port is currently allocated on fiber->gc_pool
and never destroyed.