Report IMU stream more frequently

Looks like the max rate largely depends on end receiver quality - to improve to dynamically lower if queue fills up

source/main.cpp

@@ -67,7 +67,7 @@ const unsigned long HEARTBEAT_MS         = 10000;
 const int           HEARTBEAT_DUR        = 30;
 const unsigned long BOOT_SAFE_MS         = 5000;
 #ifdef FEATURE_IMU_STREAM
-  const unsigned long IMU_STREAM_RATE_MS = 100;
+  const unsigned long IMU_STREAM_RATE_MS = 20;
 #endif
 const float BATT_FULL                    = 4.20f;
 const float BATT_EMPTY                   = 3.00f;