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.
Merge pull request #649 from jakewins/2.0-store-layer-dont-write

Move all write operations out of StoreStatementContext