protoproxy: set proxy_read_timeout to 180s

As a follow up to Ic14902d399712a1eb0d2d98ce5f505e6740ef68b, also set
protoproxy's proxy_read_timeout to be in sync with Varnish's
first_byte_timeout and PHP's max_execution_time.

Change-Id: I605ab157a3723da5663bb8fa978f16a42d67d4f1