Linearize Event Ordering In The FreeRTOS Implementation

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Background

The FreeRTOS implementation does not guarantee that all tasks see all events in the same order.

The FreeRTOS implementation posts events to subscribing tasks' queues in priority order, from highest to lowest. If a subscribing task (Sub) has a higher priority than the sending task (Send), the FreeRTOS scheduler delivers that event to Sub as soon as it enters its queue, before Send has a chance to post it to other event queues. If Sub sends events in response to the event it receives, those new events will enter other tasks' queues before those from Send even though they occurred logically after them.

Options

1. Add a lightweight mutex to ErSendEx() that forces previous calls to complete before new calls start. This uses "priority inheritance" to linearize event ordering when there are overlapping sends. This can be made to have (almost) no cost in the case where there is no contention.
2. Disable the scheduler in ErSendEx() while posting events to queues. This is simpler than 1 but costs more.