Use unique_ptr for sd_event object wrapper

Convert the sd_event object wrapper from a shared_ptr to
a unique_ptr.  Requires a new header file.

Change-Id: I868a9e88ed93878c2e0bb12e58f8d3a604510da0
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/monitor/tach_sensor.cpp b/monitor/tach_sensor.cpp
index 806eed7..80eb974 100644
--- a/monitor/tach_sensor.cpp
+++ b/monitor/tach_sensor.cpp
@@ -85,7 +85,7 @@
                        const std::string& id,
                        bool hasTarget,
                        size_t timeout,
-                       std::shared_ptr<sd_event>& events) :
+                       phosphor::fan::event::EventPtr& events) :
     _bus(bus),
     _fan(fan),
     _name(FAN_SENSOR_PATH + id),