Break mod_spdy's dependency on mod_pagespeed (issue1130001)

[mdst...@google.com]

Reviewers: hkhalil,

Message:
I think we used to use more of mod_pagespeed's code, but at this point
we're only using a single, simple utility interface class
(net_instaweb::Function). It's silly to pull in all of mod_pagespeed
just for that one class, so this change breaks our dependency on
mod_pagespeed by simply duplicating the (tiny) subset of that class's
functionality that we actually need.

Description:
Break mod_spdy's dependency on mod_pagespeed

Please review this at http://page-speed-codereview.appspot.com/1130001/

Affected files:
M DEPS
M mod_spdy/apache/apache_spdy_stream_task_factory.cc
M mod_spdy/apache/apache_spdy_stream_task_factory.h
M mod_spdy/common/executor.cc
M mod_spdy/common/executor.h
M mod_spdy/common/spdy_session.cc
M mod_spdy/common/spdy_session.h
M mod_spdy/common/spdy_session_test.cc
M mod_spdy/common/spdy_stream_task_factory.h
M mod_spdy/common/testing/async_task_runner.cc
M mod_spdy/common/thread_pool.cc
M mod_spdy/common/thread_pool.h
M mod_spdy/common/thread_pool_test.cc
M mod_spdy/mod_spdy.gyp
M net/net.gyp