param: implement RW locking

This allows concurrent read access, which are much more common; reducing
potential lock contention and increasing concurrency.

Taking a lock is expensive, and the reader lock is now even more expensive.
An RCU synchronization scheme would reduce the overhead of the readers to
increasing/decreasing an atomic counter.
Thus this should only be an intermediate step until we move towards RCU.

Tested on SITL & Pixracer.