ARROW-7844: [R] Converter_List is not thread-safe

This would trigger issues such as this:

```
Parquet file reading/writing: .........S..............Warning: stack imbalance in '.Call', 145 then 144
Warning: stack imbalance in '{', 141 then 140
Warning: stack imbalance in '{', 134 then 133
1..

══ Skipped ═════════════════════════════════════════════════════════════════════
1. write_parquet() handles various compression_level= specs (@test-parquet.R#66) - Reason: Arrow C++ not built with support for gzip

══ Failed ══════════════════════════════════════════════════════════════════════
── 1. Failure: Lists are preserved when writing/reading from Parquet (@test-parq
`object` not equivalent to `expected`.
Component “int”: Component 3: Numeric: lengths (4, 2) differ
```

Closes #6419 from fsaintjacques/ARROW-7844-r-flakyness and squashes the following commits:

99b0a0b <François Saint-Jacques> ARROW-7844:  Converter_List is not thread-safe

Authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>

r/src/array_to_vector.cpp

@@ -593,6 +593,8 @@ class Converter_List : public Converter {
 
     return Status::OK();
   }
+
+  bool Parallel() const { return false; }
 };
 
 class Converter_Int64 : public Converter {