We were storing `txid` in `ProgramState`, this meant it was impossible
to track interactive transactions. This was extracted to `Connection`
instead.
Moreover, transaction state for mvcc now is reset on commit.
Closes#2689
This not only changes schema_did_change on commit_txn for mvcc, but also
extracts the connection transaction state from non mvcc transactions to
mvcc too.
We have so many cursor types that it will be unbearable to properly make
all of them work. Let's simplify this and only focus on lazy cursor
which in the future will load from database in case we need it.
The logging code that writes out transactions to disk needs to write out
the byte array that we actually use. The code is less hairly without the
generics so drop them.
