allow only one thread to send/receive XCP commands

[danielhrisca]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

If two threads are sending XCP commands then it can happen that a thread gets the reply meant for the other one from the respQueue. Using the lock each XCP command is completed before another one can be issued