handle edge case where key might expire in the middle of the multi exec for redis store

[jmacpherson]

Discussion of the edge case: https://www.bennadel.com/blog/2785-volatile-keys-can-expire-mid-multi-transaction-in-redis-jedis.htm

Evidence of the ability to reprod with this library:

[julienwilmet]

Does this also fix #39 and #44 ?

[jmacpherson]

Does this also fix #39 and #44 ?

I can't speak to the X-RateLimit-Reset bug mentioned in #39, but it does address the ttl -1 issue, without resetting the counter back to 1 when doing so.