Shorten to one epoch per PDU

A full read cycle can take a very long time if the limits are set too
high.

servers/lloadd/connection.c

@@ -128,6 +128,11 @@ handle_pdus( void *ctx, void *arg )
             }
             break;
         }
+
+        assert( IS_ALIVE( c, c_refcnt ) );
+        epoch_leave( epoch );
+        epoch = epoch_join();
+        assert( IS_ALIVE( c, c_refcnt ) );
     }
 
     event_add( c->c_read_event, c->c_read_timeout );

servers/lloadd/epoch.h

@@ -132,6 +132,7 @@ int try_release_ref( uintptr_t *refp, void *object, dispose_cb *cb );
  */
 #define RELEASE_REF( object, ref_field, cb ) \
     do { \
+        assert( IS_ALIVE( (object), ref_field ) ); \
         if ( !__atomic_sub_fetch( \
                      &(object)->ref_field, 1, __ATOMIC_ACQ_REL ) ) { \
             epoch_append( object, (dispose_cb *)cb ); \