Contended CAS on every buffer allocation

• Originally, pre open-sourcing, the code used folly::Singleton::try_get, which is not available in the latest stable version of folly and incurs a contended CAS on every buffer allocation.
• This have been replaced with folly::Singleton::get to fix build, but that change broke tests (why?) when compiling with CMake.
• Once replaced back with get_weak().lock(), we again have a contended CAS on buffer allocation path. * For this reason, I'd vote to get rid of Singleton hack and inject a Singleton instance into frame serialisers.

Actually, it's not that. FrameBufferAllocationTest is broken, as it leaves Singleton in invalid state. The next test that uses allocator (RequestChannel) crashes... That wasn't very obvious...

Closing this, as I will leave ::get usage.