Add a MongoDB index for time based queries

Hello

I realized that for purely time based queries, like Thruk's alerts history, we didn't use an index, so I added one. The performance increase is very good: from more than 1 second per query, I get about 80ms now.

I tested the maximum Thruk's pages I could, analysed resulting queries and they all use an index. I couldn't test with other front-ends.

The best way to do it would be having an exhaustive list of possible livestatus api queries, launch them and make sure they all use an index, but this is a start and I hope that with the two indexes we will cover almost all queries.