Move all write operations out of StoreStatementContext

This moves all write operations out of StoreStatementContext, and clarifies it's role as a read-only part of the stack.

Instead, writes are handed over to the old transaction infrastructure at the TransactionStateStatementContext, by it talking to TxState, which in turn delegates to PersistenceManager.

This is an initial step towards concentrating ownership of transaction state into one single place. Even though it still delegates back into the old infrastructure, combining it behind one abstraction will help refactoring further.