The nullify potential unsafe (in security mind) functions was moved from box.lua
to separate functions which calls after init.lua. It allows call require,
os.execute, and another unsafe function in init.lua.

Add Request object with method execute. Allocate
Requests on fiber->gc_pool. Clear txn from request data.
Make txn commit/rollback algorithm independent on the actual
request or request data.
Update valgrind suppressions with new call paths.

Rename a global variable, which had too common name,
to avoid easy to miss bugs.

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.

We needed to have txn_cleanup() only because a tuple
which was read in txn was used in fiber->io_vec.
Decouple txn and network I/O and make sure
we can commit/rollback txn whenever we like, regardless
of what is given to fiber->iovec.

@todo: perhaps we need to optimize performance
of port cleanup for big selects.

Rename classes in *.rl file.
Patch valgrind suppressions.

We don't use box_ prefix consistently, so don't use it at all.

Remove box_ prefix from box_tuple and box_txn.

Move out from box.m INSERT/UPDATE/SELECT/DELETE/CALL
implementation.

Cherry-picking code from txn-refine.
Remove an extra member in struct box_txn.
Rename txn->tuple to txn->new_tuple.