[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
[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
Optimizations...
Add JVSTM/DSI Backend
[jvstm-common] JVSTMDomainObject now stores its external id, as well as its oid.
[long-tx-api] Caching DomainSlotKey's hashCode
[long-tx-api] Refactor: ReadSet now allows for batch insertion of elements
[jvstm-common] Only validate Long Lived Transaction if anything was written
[long-tx-api] Refactor: WriteSet is now kept as a ValueType
WriteSet is now split
WriteSet now returns a proper map
[jvstm-common] Bug Fix: Generate correct slot name for collection boxes
[long-tx-api] Don't create a WriteSet node if the current node still has room
[long-tx-api] Don't store the contents in a String, store them in a JsonElement instead
[jvstm-common] Support LongLivedTransactions using direct JsonElement
[bplustree] DomainBPlusTree now serializes to JsonElement instead of String
[long-tx-api] Performance Improvements
[long-tx-api] Don't duplicate the WriteSet map, instead put the entries in the new map.

  * Also allow for the Maximum WriteSet size to be configured
[jvstm-common] When Aborting a Long-Lived Transaction, also abort the underlying