REORG: spoe: Move struct and enum definitions in dedicated header file

SPOA example has been Updated accordingly
diff --git a/contrib/spoa_example/Makefile b/contrib/spoa_example/Makefile
index aee6139..3105ef0 100644
--- a/contrib/spoa_example/Makefile
+++ b/contrib/spoa_example/Makefile
@@ -7,7 +7,7 @@
 
 CFLAGS  = -g -O2 -Wall -Werror -pthread
 LDFLAGS = -lpthread  -levent -levent_pthreads
-INCS += -I../../include
+INCS += -I../../include -I../../ebtree
 LIBS =
 
 OBJS = spoa.o