Python: Add a DelayedCallGuard to xbmc.executeJSONRPC.

Avoids locking other python threads running on the same process space as XBMC, since executeJSONRPC may trigger activity on them.
Merge pull request #3202 from mazkolain/python-jsonrpcfix

Avoid locking other Python threads when calling xbmc.executeJSONRPC

Fixes a bug. Can't hurt anything so it's going in.