Ensure logs don't indefinitely append past capacity

Summary:
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There could be cases, albeit unlikely, that logs could be appended for the current state indefintely that would ultimate fail due to not having enough memory.

This change puts a cap on that.

Reviewed By: mweststrate

Differential Revision: D42313904

fbshipit-source-id: 7fd96be822c9427720bccb41c6c32a39213c7652

xplat/Flipper/FlipperState.h

@@ -10,6 +10,7 @@
 #include <map>
 #include <memory>
 #include <mutex>
+#include <sstream>
 #include <string>
 #include <vector>
 
@@ -53,10 +54,11 @@ class FlipperState {
   void success(std::string);
   void failed(std::string, std::string);
   void started(std::string);
+  void ensureLogsCapacity();
 
   std::mutex mutex; // Protects all our member variables.
   std::shared_ptr<FlipperStateUpdateListener> mListener = nullptr;
-  std::string logs;
+  std::stringstream logs;
   std::vector<std::string> insertOrder;
   std::map<std::string, facebook::flipper::State> stateMap;
 };