Memtx engine doesn't allow yielding inside a transaction. To achieve
that, it installs fiber->on_yield trigger that aborts the current
transaction (rolls it back, but leaves it be so that commit fails).

There's an exception though - DDL statements are allowed to yield.
This is required so as not to block the event loop while a new index
is built or a space format is checked. Currently, we handle this
exception by checking space id and omitting installation of the
trigger for system spaces. This isn't entirely correct, because we
may yield after a DDL statement is complete, in which case the
transaction won't be aborted though it should:

  box.begin()
  box.space.my_space:create_index('my_index')
  fiber.sleep(0) -- doesn't abort the transaction!

This patch fixes the problem by making the memtx engine install the
on_yield trigger unconditionally, for all kinds of transactions, and
instead explicitly disabling the trigger for yielding DDL operations.

In order not to spread the yield-in-transaction logic between memtx
and txn code, let's move all fiber_on_yield related stuff to txn,
export a method to disable yields, and use the method in memtx.