Queue processor may use x-server-timestamp set by another gateleen instance

[lbovet]

When a gateleen instance forwards to another one, the original x-server-timestamp header is used to calculate the expiration time in QueueProcessor. In case of delay or unsynchronized clocks, the expected TTL is not respected.

Proposal: add another header that would only be used internally by the queue handler. It will be set at enqueue time and removed when the request is played: x-queue-timestamp
This header would never be forwarded.
So we decouple the x-server-timestamp header and queue behaviour.