Add a wrapper around the PAK-file DDC backend which can compress cache entries. Big reduction in the size of the DDC pak file, and seems to add negligable amount (measured average < 0.5s) to startup time. For UE-8823.
[CL 2449854 by Matthew Griffin in Main branch]
Fix for building engine DDC on Mac; suspect that lock is due to recursion in FDerivedDataBackend::Get(), due to recursion through WaitForQuiescence. Instead of appending existing cache on startup, append it on shutdown if a command line parameter is present instead.
[CL 2426046 by Ben Marsh in Main branch]
Add a parameter to the pak file backend to merge DDC with an existing cache, and include DDC for MobileStarterContent and templates in Rocket build.
#codereview Matthew.Griffin
[CL 2425921 by Ben Marsh in Main branch]
