Merge pull request RichardKnop#174 from xitongsys/master

fix err chan size in server.go

Merge pull request RichardKnop#173 from RichardKnop/feature/fix-send-…

…group-concurrency

Fix pool logic to handle concurrency in SendGroup call.

When using AMQP as backend, set x-expires parameter so task state que…

…ues will eventually be cleaned up.

Formatting fixes for v1/backends/async_result.go