Review thread-safety and develop high-concurrency test

[maxirmx]

We also want to support threaded processing, i.e. concurrent access to the tebako memfs.

For example, ruby-packer fails (segfaults) on threads when there is more than one file IO on a single file (probably due to libsquashfs not being threadsafe).
https://github.com/metanorma/packed-mn/issues/129

_Originally posted by @ronaldtse in https://github.com/tamatebako/tebako/issues/45#issuecomment-991431512_