Clear entire cache when it fills.

- Assumes situations where the cache fills likely means the
  cache isn't being used that efficiently anyway -- and deleting
  one entry each time the cache gets a new entry once filled
  will be slow.

Author: dlongley

lib/context.js

@@ -647,8 +647,11 @@ api.getInitialContext = (options) => {
     getInverse: _createInverseContext,
     clone: _cloneActiveContext
   };
+  // TODO: consider using LRU cache instead
   if(INITIAL_CONTEXT_CACHE.size === INITIAL_CONTEXT_CACHE_MAX_SIZE) {
-    INITIAL_CONTEXT_CACHE.delete(INITIAL_CONTEXT_CACHE.keys().next().value);
+    // clear whole cache -- assumes scenario where the cache fills means
+    // the cache isn't being used very efficiently anyway
+    INITIAL_CONTEXT_CACHE.clear();
   }
   INITIAL_CONTEXT_CACHE.set(key, initialContext);
   return initialContext;