fix for #796, #678

src/box/engine.cc

src/box/engine.h

src/box/sophia_engine.cc

src/box/sophia_engine.h

src/box/txn.cc

test/long_run/box.lua

test/long_run/memtx_delete_replace_update.result

test/long_run/memtx_delete_replace_update.test.lua

test/long_run/sophia_delete_replace_update.result

test/long_run/sophia_delete_replace_update.test.lua

test/long_run/suite.ini

test/sophia/conflict.lua

test/sophia/conflict.result

test/sophia/conflict.test.lua

test/sophia/suite.ini

third_party/sophia