This commit introduces several optimizations to write operations in `HazelcastSessionRepository`.
- when storing a new session, `IMap#set` is now used instead of `IMap#put`
- when updating an existing session, `IMap#executeOnKey` and a dedicated `EntryProcessor` are used
To make these two changes possible, internal `HazelcastSession` now adds a flag to determine which of the two mentioned write scenarios to use, and also tracks a delta of session attributes in order to optimize updates.
Closes gh-850
f5912da089