EKF: RingBuffer minor improvements

- allocate IMU and output buffers on construction according to defaults
 - determine buffer max time delay based on configuration parameters
 - reorder flowSample and extVisionSample to minimize padding
 - adjust parameter defaults to match PX4-Autopilot

test/test_EKF_ringbuffer.cpp

@@ -62,7 +62,6 @@ class EkfRingBufferTest : public ::testing::Test {
 
 	void TearDown() override
 	{
-		_buffer->unallocate();
 		delete _buffer;
 	}
 };