Handle expired limit and reset

[promag]

This fixes an edge case when using ioredis:

• get() executes mget which returns count, limit and reset;
• suppose count is expired and the other 2 are about to expire;
• since count is undefined then it will call create();
• create() will set limit and reset but it won't work because of NX (keys still exist);
• limit and reset expire, now only count is set.
• the next get() will call decr() with undefined limit and reset which causes a loop.

There is no risk to overwrite limit and expire set by other session because, in that case, multi/exec would abort all sets due to the watched count.

[noamshemesh]

Thanks. I wont be near a computer until next week

[promag]

I've pushed an alternative implementation in #33.