folly/singleton: fatal in unrecoverable error cases

folly/singleton: fatal in unrecoverable error cases

Summary: Early in the startup process there may not be a default signal handler installed, and stack traces are not available; also during the startup process is when init-order fiascos occur.  Dump a stacktrace and fatal when an unregistered singleton is used.

Also fatals -- with glog `LOG(FATAL)`, which triggers an ABRT signal -- on other illegal and unrecoverable usage like double-registration or circular dependency.

Reviewed By: @andriigrynenko

Differential Revision: D2200408