gh-698: test and related fixes

* Fix gh-698: duplicate-key error followed by segfault

* Add long_run test suite, which include test case from the
ticket for memtx and sophia engines.
Result files must be zero-diff

Test script also has been fixed in a number of ways:
  a) crud operation on sophia space always return nil
  b) sophia_space:len() is not monotonous
  c) update of primary key is not allowed

* Fix return of vclock_sum() as wal_writer() result, reuse
it for txn->signature

* Fixed incorrect tuple guard usage in execute_delete()

* Fixed sophia delete operation: return old_tuple as an
exception for SophiaIndex::replace(). Do not skip
wal_write() and correctly set txn->signature

* Put engine commit code after wal_write(), reuse proper
txn->signature. Engine->commit yet can throw an exception
on concurrent transaction conflict or error

* rollback sophia transaction on conflict

* add second test for gh-698; rename and disable zlong_run tests

* make rollback statement engine specific

* make txn_finish() engine specific; use it both by commit/rollback to gc

* refactor execute_delete()

* upgrade sophia submodule: efficient msgpack storage

* engine api: style fixes