shm: add APIs for implementing keep-alive process

fairmq/shmem/Common.cxx

@@ -12,6 +12,7 @@
 
 #include <unistd.h>
 
+#include <iomanip>
 #include <sstream>
 #include <string>
 
@@ -44,5 +45,12 @@ uint64_t makeShmIdUint64(const std::string& sessionId)
     return id;
 }
 
+std::string makeShmIdStr(uint64_t val)
+{
+    std::stringstream ss;
+    ss << std::setfill('0') << std::setw(8) << std::hex << val;
+    return ss.str();
+}
+
 
 }   // namespace fair::mq::shmem