Potential deadlock in PCRE cache

In 2014 I improved HHVM's PCRE compilation cache, introducing a new [thread-safe container](https://github.com/facebook/hhvm/blob/master/hphp/util/concurrent-lru-cache.h) to hold cache entries. I also published the container class as a separate library. Another user of the library reported a deadlock issue, and it appears that it may also affect HHVM. If an application uses many different regexes and traffic is heavy, HHVM may globally deadlock. I'm not an HHVM user anymore, but out of courtesy I am forwarding the issue. The deadlock issue is discussed at https://github.com/tstarling/thread-safe-lru/issues/5 and the proposed fix is https://github.com/tstarling/thread-safe-lru/pull/6 .