CaChannel: investigate behaviour with multiple threads

As a developer I believe I have seen a segmentation fault / heap corruption issue while running python code which looks like:

```
from CaChannel import CaChannel, ca
import threading

c = CaChannel("TE:NDW1799:DAE:RUNSTATE") # A string PV
c.search_and_connect(None, lambda *a, **k: None, None)

threads = [threading.Thread(target=c.getw, args=(ca.DBR_STRING, )) for _ in range(1000)]

for thread in threads:
thread.start()

for thread in threads:
thread.join()

print("Exited normally, crash did not occur.")
```

My suspicion is that this is a bug in CaChannel but I'm not completely sure. This caused an issue for LET (see https://github.com/ISISComputingGroup/IBEX/issues/4102 ).

Acceptance criteria:
- [ ] Cause of bug is understood
- [ ] Bug is fixed (or raised to the author of CaChannel if appropriate)