convert null terminator in ascii dump

Summary:
The ASCII output is almost always useless to me as the first '\0' byte in the key or value causes it to stop printing. Since all characters are already surrounded by spaces, "\ 0" (how we display a backslash followed by a zero) and "\0" (how this PR displays a null terminator) are distinguishable. My assumption is the value of seeing all the bytes outweighs the value of the alignment we had before, where we always had one character followed by one space.
Closes https://github.com/facebook/rocksdb/pull/3203

Differential Revision: D6428651

Pulled By: ajkr

fbshipit-source-id: aafc978a51e9ea029cfe3e763e2bb0e1751b9ccf

table/block_based_table_reader.cc

@@ -2423,11 +2423,19 @@ void BlockBasedTable::DumpKeyValue(const Slice& key, const Slice& value,
   std::string res_key(""), res_value("");
   char cspace = ' ';
   for (size_t i = 0; i < str_key.size(); i++) {
+    if (str_key[i] == '\0') {
+      res_key.append("\\0", 2);
+    } else {
       res_key.append(&str_key[i], 1);
+    }
     res_key.append(1, cspace);
   }
   for (size_t i = 0; i < str_value.size(); i++) {
+    if (str_value[i] == '\0') {
+      res_value.append("\\0", 2);
+    } else {
       res_value.append(&str_value[i], 1);
+    }
     res_value.append(1, cspace);
   }