Named axis is not thread-safe

### Version of Awkward Array

HEAD

### Description and code to reproduce

Named axis uses this class-level dictionary:
https://github.com/scikit-hep/awkward/blob/a7f7d447a64f7e19aff0f488ebf6b4e9220ca850/src/awkward/_namedaxis.py#L32
which is not thread-safe