memtx: vocally abort a transaction in case of implicit yield

Before this patch, memtx would silently roll back a multi-statement
transaction on yield, switching the session to autocommit mode.

It would do nothing in case yield happened in a sub-statement
in auto-commit mode.

This could lead to nasty/painful to debug side-effects in
malformed Lua programs.

Fix by adding a special transaction state - aborted, and enter
this state in case of implicit yield.

Check for what happens when a sub-statement yields.
Check that yield trigger is removed by a rollback.

Fixes gh-2631
Fixes gh-2528