Fix memory leak and improve performance

[MP0w]

1. unsubscribe didn't remove the index from the map
2. subscribe needed to create an array of all keys to get the next index
3. the map used arrays to store all indexes, replaced with a Set for faster lookup

• Added tests before the fix to verify it still works fine
• Tested performance of adding 100K subscribers and publishing a message:
  new implementation: 85ms
  old implementation: 120 sec

[MP0w]

must have closed it by mistake