Optimize getting the latest events in each room

[S7evinK]

As mentioned internally, this significantly speeds up getting the latest event in each room provided.

Output of EXPLAIN ANALYSE

 Planning Time: 0.408 ms
 Execution Time: 9158.038 ms

for the original with a cold cache

 Planning Time: 0.296 ms
 Execution Time: 101.383 ms

with a warm cache.

The optimized query with a cold cache:

 Planning Time: 1.902 ms
 Execution Time: 3.752 ms

and with a warm cache:

 Planning Time: 0.425 ms
 Execution Time: 0.131 ms

[kegsay]

Thanks for this!