Update state machine

[gitbuda]

No description provided.

[gitbuda]

Closes #28

• Added InTransaction status.
• Checking preconditions with match + Result seems quite good to me because it's contained in one block. There is a difference between https://github.com/memgraph/pymgclient/blob/master/src/connection-int.c#L77 ("internal" error handling) and https://github.com/memgraph/pymgclient/blob/master/src/cursor.c#L156 ("public" error handling). It seems to me that Rust's pattern matching + Result combines everything in one place. Long story short, I would leave that as is (the Rust structure), but we should discuss 😄 to make lasting changes.
• There is some difference in the behavior at this point, that's for the discussion as well.
• Connection::pull_all is a private method used in one place, it could be inlined, but that's more-less equivalent.
• In the Python case, the cursor is there because of https://www.python.org/dev/peps/pep-0249/. In the Rust case, that clearly defined API doesn't exist. I would keep that simple for now.

[gitbuda]

@antaljanosbenjamin I think this is finally ready for review 😄

[antaljanosbenjamin]

I created two issues (#34 and #35) regarding to our discussion yesterday. There are two minor comments from me, but it looks really nice. The status management become much readable.