utility/timer: Implement

We often need a continually ticking timer for our daemons. This utility
wraps an sd_event time source as a convenience. This is meant to be a
usable replacement for the timer.hpp found in other openbmc projects.

Tested:
    New tests pass with full coverage. Changes to the phosphor-watchdog
    that rely on this utility work as expected.

Change-Id: Id12aed9e5b018e7eca825c4a7ac7b4f46e2f04c6
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/Makefile.am b/src/Makefile.am
index 518ac1f..9cd7706 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -40,3 +40,6 @@
 libsdeventplus_la_SOURCES += sdeventplus/source/time.cpp
 
 nobase_include_HEADERS += sdeventplus/test/sdevent.hpp
+
+nobase_include_HEADERS += sdeventplus/utility/timer.hpp
+libsdeventplus_la_SOURCES += sdeventplus/utility/timer.cpp