Add shared mutex field to IODebugContext (facebook#13993)

Summary:
There can be concurrent reads/writes to fields in `IODebugContext`. One example we have seen is for the `cost_info` field which is of type `std::any`. In fact, in RocksDB's async MultiRead implementation, the same `IODebugContext` is re-used across separate async read requests.

Pull Request resolved: facebook#13993

Test Plan: Update code which reads/writes to `cost_data` to first acquire shared/exclusive lock on the `mutex` field. There should not be any race conditions when async MultiRead is used.

Reviewed By: pdillinger

Differential Revision: D83091423

Pulled By: archang19

fbshipit-source-id: 4db86d33cf162ed39114b1cd115fcd8964c8ff9b

Author: archang19

include/rocksdb/file_system.h

@@ -24,6 +24,7 @@
 #include <functional>
 #include <limits>
 #include <memory>
+#include <shared_mutex>
 #include <sstream>
 #include <string>
 #include <unordered_map>
@@ -254,6 +255,10 @@ struct IODebugContext {
   // Arbitrary structure containing cost information about the IO request
   std::any cost_info;
 
+  // FileSystem implementations can use this mutex to synchronize concurrent
+  // reads/writes as needed (e.g. to update the counters or cost_info field)
+  std::shared_mutex mutex;
+
   IODebugContext() {}
 
   // Copy constructor