* Adds the send_once command and message type, which allows a user to request a variable to be sent immediately and only once (intended to replace the var_add, var_send, var_clear idiom that is commonly used for this purpose)
* Minor refactoring of variable server internals to reduce repeated code
* Adds SIM_test_varserv to integration test to test basic variable server functionality
* Changes graphics client for SIM_billiards to use var_send_once as an example of intended use
* Add documentation for var_send_once in docs and tutorial
* Set exit_code_enabled in trick unit tests to be true by default
* Patch for failing bookworm build
