Facilitate Multi-Threaded Access

[garethpotter]

Add a request per-context and a reader-writer lock to facilitate uncontended concurrent reads.

[martinjaeger]

This is quite a huge breaking change (it touches almost every function).

Can you please give a bit more context under which circumstances this is needed? Most importantly: Do we really need the struct thingset_global_context to facilitate a readers-writer locking mechanism?

[martinjaeger]

These member variables should be documented.

[garethpotter]

I'm going to close this because I think we're going to adopt another approach.

For posterity though, my original rationale for it was based on a theory more than anything else that if a device is being read (or is publishing) more than it is being written to (probably usually the default?), there is no need for read access to be mutually exclusive, hence the reader-writer lock. To make that work, the buffers involved obviously need to become per-operation rather than global, so I:

• repurposed the thingset_context as a sort of per-operation context
• made the truly global parts (i.e. the object array, the replacement for the mutex lock) of that struct part of thingset_global_context.