Sergei Fedorov

Add dependent libraries via ExternalProject

Use ::boost (::std) ::variant for storing substates of state machines that do not have history.

State machines that do not defer events should not use a deferred queue

Refactor code

1

Pushdown stacks Nested state containers

Parse the non-select result. Add the information to the `resultset` interface.

Rename the top namespace to `psst`, move top include directory to `pushkin`

Use callbacks with `::std::exception_ptr` for more convenient usage in conjunction with other asynchronous libraries.

Do not use the logger if logging is off.

Use zmij/afsm#7 feature to prioritize events. Commit/rollback should have a lower priority than `execute` or `execute_prepared` so that queries started on other queries' completion callback execute before `commit` that was...