Refine ContextManager

- Add isManagingContext API
- Remove global lock and global variable (use Singleton instead)
- Encapsulate getSlotIndexFromAddress API
- Extract configurable define to Config.h
diff --git a/lib/bcc/ScriptCached.cpp b/lib/bcc/ScriptCached.cpp
index 1003163..183e552 100644
--- a/lib/bcc/ScriptCached.cpp
+++ b/lib/bcc/ScriptCached.cpp
@@ -30,7 +30,7 @@
 ScriptCached::~ScriptCached() {
   // Deallocate the bcc script context
   if (mContext) {
-    deallocateContext(mContext);
+    ContextManager::get().deallocateContext(mContext);
   }
 
   // Deallocate string pool, exported var list, exported func list