Use a smoother running average for avg_ttl in INFO.
Use a smoother running average for avg_ttl in INFO.

Reported here:
https://www.reddit.com/r/redis/comments/42r0i0/avg_ttl_varies_a_lot/

janekmi/redis,pedigree/redis,4396/redis,janekmi/redis,pedigree/redis,xuguruogu/redis,xuguruogu/redis,xuguruogu/redis,janekmi/redis,pedigree/redis,janekmi/redis,xuguruogu/redis,4396/redis,4396/redis,4396/redis,pedigree/redis