gh-306: implement automatic transaction rollback on yield.

src/box/engine.cc

src/box/engine.h

src/box/engine_memtx.cc

src/box/key_def.h

src/box/txn.cc

src/box/txn.h

src/errcode.h

test/box/misc.result

test/box/transaction.result

test/box/transaction.test.lua