This class gracefully handles concurrent modifications and auto-retries but
embeds no tool to rollback.
+ Transactions may register ``subtransactions``. This field is a list of
+ ``Transaction``. Sub-transactions are played after the main transactors
Transactions status may be persisted for tracability, further analysis...
for this purpose, a minimal schema is embedded in this base class. When
- modify the target object according to the transaction's parameters.
- save the (modified) target to the DB
+ - run sub-transactions (if any)
- save the transaction to the DB
Each transation may be retried up to ``MAX_RETRIES`` times automatically.