Add support for transaction chaining.

[je-ik]

When multiple attributes with different types of TransactionModes are part of the same transaction, we need to delegate the commit to individual attributes to sub-transaction managers. The transaction will happen with the highest mode from all attributes, but the top-most manager will verify only integrity of the highest-level mode attributes. The lower level need to be delegated to appropriate managers. All calls (open, update, commit, abort) then have to follow this algorithm:

• perform the operation within the current transaction manager, if the operation fails
  • if there are any open downstream transactions, abort them
  • return failure to the client
• otherwise, check if there are any downstream managers
• if yes, send the operation (open, update, commit, abort) to all downstream managers and gather responses
  • if any fails, return failure, rollback all
  • otherwise, return success to the client

Subtask of #198

[je-ik]

Keep all open downstream transactions within state of the current transaction.