More performant index

[Peter Macherey]

I'm having an issue:

I observed that after that the interval query generates quite a lot of load on the database after a while in case the keepHistory option is enabled. The problem is that the query in the __send function is triggering the compound index which first index sentAt is a timestamp. Because of the timestamp the database always needs to search a lot of index keys before returning the reults.

I have a suggestion:

I would suggest putting the isSent index as the first index of the compound index.

[Peter Macherey]

I added a PR #35

[dr.dimitru]

@PeteMac88 published as v1.3.4