MB-17901: Add custom header for request timeout in pluggable UI
MB-17901: Add custom header for request timeout in pluggable UI

Requests to services using the pluggable UI can add a custom header
"ns-server-proxy-timeout" to specify a request timeout when the request
is proxied to the server handling the service.

The header is removed before forwarding the request. If the
request times out a 504 Gateway Timeout response will be given.

Change-Id: I9d0f7a6c702db1c609380434efc43e0ee1bc758a
Reviewed-on: http://review.couchbase.org/59569
Tested-by: Anders Nygren <4f8bff4ce020de40cfee5db6ca7fd51acc06e5c4@couchbase.com>
Reviewed-by: Dave Finlay <7b10a9ed9f70e64c30b07607a671ae61c33f14c8@couchbase.com>

ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server