At the moment we should have 3 possible options to check return state
during vm context unloading:
* no check
* ensure the stack is empty
* ensure the stack is not empty
It is necessary to distinguish them because new _deploy method shouldn't
left anything on stack. Example: if we use _deploy method before some
ordinary contract method which returns one value. Without these changes
the contract invocation will fail due to 2 elements on stack left after
invocation (the first `null` element is from _deploy, the second element
is return-value from the ordinary contract method).
This was done in https://github.com/neo-project/neo/pull/1693
for native calls. `OnPersist` script still uses `DROP` though
as value is pushed via `CheckReturn` logic for regular calls.
