No retries when adding a purposely removed sensor

When a sensor is removed due to a removal return code being received
during a read, no read retries should be done when attempting to re-add
it. This requires the sensor to be successful in its first read
attempted when being re-added and keeps the re-adding of purposely
removed sensors from consuming up to 1 second during the re-add attempt.

Tested:
    A removed sensor is attempted to be read once on re-add

Change-Id: Ia7eb463deb569c9d10883632e017b4dd05413854
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
diff --git a/mainloop.hpp b/mainloop.hpp
index d1fe9b0..9a51580 100644
--- a/mainloop.hpp
+++ b/mainloop.hpp
@@ -14,6 +14,7 @@
 
 using Object = std::map<InterfaceType, std::experimental::any>;
 using ObjectInfo = std::tuple<sdbusplus::bus::bus*, std::string, Object>;
+using RetryIO = std::tuple<size_t, std::chrono::milliseconds>;
 
 /** @class MainLoop
  *  @brief hwmon-readd main application loop.