Revert "Revert "locking: support shared memory locking""

This reverts commit be9a78b18a8d415377cbcd547b24a62d99f94955.
shared-memory-locks: Fix a segmentation fault

Do not instantiate a ProxyFetchFactory when the ServerContext is
NULL on cfg_s
Merge pull request #158 from pagespeed/oschaaf-shared-mem-locks-fixed

Shared mem locks fixed