[long-tx-api] Improved the API to add elements to the read/write sets.

  * Adapted LongLivedTransaction to use the new API
  * Renamed attribute to slotName in LogEntry
[long-tx-api] Added utility print method to LogEntry
[jvstm-common] JSON internalizers and externalizers now receive the slot name instead of the VBox
[jvstm-common] Added a method to JVSTMDomainObject to return a VBox by its name

  * Also uniformized the format of the generated code for JSON handling
[jvstm-common] Proper support for Transactional Context commit
[long-tx] The version of a TransactionalContext is now filled when the first step executes