bug: reference counter increment/decrement not atomic

[t-bre]

Description

Incrementing and decrementing of the reference count in rtcan_msg_t is not atomic and therefore not thread safe.

Expected Behaviour

Reference count implemented in thread-safe manner.

Additional Notes

• Could use a semaphore for each RTCAN message, though there is a large memory cost there.
• Could also have a "reference count mutex" which locks ALL reference counts, this is less costly in terms of memory but could slow things down.

[t-bre]

Possibly useful: https://nullprogram.com/blog/2015/02/17/

We are building with C11, so we should be able to make use of stdatomic.h.

[t-bre]

stdatomic.h has been added as part of #18. The relevant documentation is here.

I think what we need to do is:

• Change the type of the reference count to atomic_uint.
• Use atomic_store to initialise / set the reference count.
• Use atomic_load to read the reference count.
• Use atomic_fetch_add to increment the reference count.
• Use atomic_fetch_sub to decrement the reference count.

[kruti99]

yes, working on it. Thanks!