Solve problem with sc-memory crash due to access to deleted element from another thread

[ShunkevichDV]

Approximate description of the problem:
There is an element that belongs to several sets
The first agent iterates over the sets and does some additional checking (does not delete anything)
The second agent iterates over the sets and, having found the required set, removes the membership arc between the element and this set
The first agent crashes because of this

[Vikort]

sc-memory crash test

https://github.com/Vikort/sc-machine/tree/bug/sc-memory-crash-test

To run tests

• Clone repository

  git clone https://github.com/Vikort/sc-machine.git
  cd sc-machine
  git checkout bug/sc-memory-crash-test
  git submodule update --init --recursive

• Build sc-machine

  In sc-machine folder

  mkdir build
  cd build
  cmake ..
  make

• Run

  In sc-machine folder

  cd bin
  ./sc-memory-crash