common,filestore: silence GCC-8 warnings

should catch polymorphic exceptions by reference. this silences warnings
like:

error: catching polymorphic type ‘class std::RetryException’ by value
[-Werror=catch-value=]
     } catch (std::RetryException) {

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/common/cmdparse.h

@@ -54,7 +54,7 @@ cmd_getval(CephContext *cct, const cmdmap_t& cmdmap, const std::string& k, T& va
     try {
       val = boost::get<T>(cmdmap.find(k)->second);
       return true;
-    } catch (boost::bad_get) {
+    } catch (boost::bad_get&) {
       handle_bad_get(cct, k, typeid(T).name());
     }
   }

src/mgr/DaemonState.h

@@ -130,7 +130,7 @@ class DaemonState
       auto p = config_defaults_bl.begin();
       try {
 	decode(config_defaults, p);
-      } catch (buffer::error e) {
+      } catch (buffer::error& e) {
       }
     }
     return config_defaults;

src/os/filestore/LFNIndex.h

@@ -63,7 +63,7 @@
       out:					\
       complete_inject_failure();		\
       return r;					\
-    } catch (RetryException) {			\
+    } catch (RetryException&) {			\
       failed = true;				\
     } catch (...) {				\
       ceph_abort();				\