SERVER-43074: Make appendMultikeyPathsAsBytes threadsafe.

The method previously relied on catalog level serialization for guaranteeing
serialized access to a global variable.

Author: Daniel Gottlieb

src/mongo/db/storage/bson_collection_catalog_entry.cpp

@@ -42,7 +42,6 @@ namespace {
 // We use that value to represent the largest number of path components we could ever possibly
 // expect to see in an indexed field.
 const size_t kMaxKeyPatternPathLength = 2048;
-char multikeyPathsEncodedAsBytes[kMaxKeyPatternPathLength];
 
 /**
  * Encodes 'multikeyPaths' as binary data and appends it to 'bob'.
@@ -54,6 +53,8 @@ char multikeyPathsEncodedAsBytes[kMaxKeyPatternPathLength];
 void appendMultikeyPathsAsBytes(BSONObj keyPattern,
                                 const MultikeyPaths& multikeyPaths,
                                 BSONObjBuilder* bob) {
+    char multikeyPathsEncodedAsBytes[kMaxKeyPatternPathLength];
+
     size_t i = 0;
     for (const auto keyElem : keyPattern) {
         StringData keyName = keyElem.fieldNameStringData();