Change exception tracer to use per-thread caching in libunwind

Summary: Because the global cache is slow and contends on locks.

Test Plan: testinproduction

Reviewed By: philipp@fb.com

FB internal diff: D1081259

folly/experimental/exception_tracer/ExceptionTracerLib.cpp

@@ -81,6 +81,8 @@ void initialize() {
       !orig_cxa_end_catch || !orig_rethrow_exception) {
     abort();  // what else can we do?
   }
+
+  initStackTrace();
 }
 
 }  // namespace

folly/experimental/exception_tracer/StackTrace.h

@@ -75,6 +75,11 @@ void clearStack(StackTraceStack** head);
  */
 int moveTop(StackTraceStack** from, StackTraceStack** to);
 
+/**
+ * Initialize the stack tracing code.
+ */
+void initStackTrace();
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif