ats_pagespeed: Support In-Place-Resource-Optimization (TS-2989)

With this, ats_pagespeed support In-Place-Resource-Optimization
(IPRO) and the new OptimizeForBandwidth mode, which targets CDN's
and shared hosting scenarios.

Also, see:
https://developers.google.com/speed/pagespeed/module/optimize-for-bandwidth
TS-3033 Test for FD_SETSIZE, since Ubuntu detects the overflow
Doc: Remove non-existent directory and add atscppapi to Doxygen configuration.
docs: Add stubs for references that currently contribute warnings to the Sphinx build
TS-2832: Add links from an API description to the code for that object
TS-3039: plug BIO leak in OCSP stapling support
TS-3041: Add traffic_layout to .gitignore
docs: Make the version info identical when building with Autotools (e.g. $ make html) and without (e.g. on Read the Docs)
docs: Rename Doxyfile.in -> Doxyfile to use it on Read the Docs.  Enable SOURCE_BROWSER and disable STRIP_CODE_COMMENTS to generate highlighted source code like the sphinx.ext.viewcode extension does.
TS-2832: Doxygen can't create output directory if doc/source directory doesn't exist.  Use the default XML_OUTPUT tag instead.
TS-2832: Parse Doxygen XML output with the Python lxml library
TS-2832: Add highlighted source code to the Read the Docs output directory
docs: Doxygen can't create output directory if doc/_build/html directory doesn't exist, so add it to Git
Clarify changes that are to the docs only
TS-2832 docs: Fix links to source code for e.g. a struct
TS-2832 docs: Fix build failure
TS-2832 docs: Fix links to source code
docs: Update Doxyfile to get rid of obsolete tag warnings
docs: Include apidefs.h.in in the files parsed by Doxygen
docs: The highlighted source files are *huge*, disable SOURCE_TOOLTIPS