minor test enhancements
Optimize clear of repeated fields

Now repeated message fields are stored in RepeatedField struct, that is
similar to c++-protobuf RepeatedField. Clear operation of RepeatedField
does not invoke destructors of contained messages, thus allocated fields
of cleared messages are not deallocated.

This patch speeds up test_repeated_messages twice.

Note that unlike C++, messages are stored in RepeatedField, not in
RepeatedPtrField.  I believe, Google chose RepeatedPtrField due to safety
issues with RepeatedField in C++.