Modify ack deadline of delayed messages according to remaining time

[anaspk]

Suggested changes for #41

[v8-ict]

Hi @anaspk

Not sure, but i think this code will now break, using larger delays, as google sets an absolute limit on AckDeadline. i would expect that it will throw an exception, if you delay a job for example 4,8 or even 24 hours.

Saying, cannot set ackDeadline to "TO LARGE VALUE"

p.s. i am not a maintainer, just using this package for a while now, and following it.