too many files of oap-server.log without deletion automatically

[AngryMills]

oap-server runs online for 2 months,and make 81G logs.
<DefaultRolloverStrategy> of log4j2.xml is not efficient.
Can add a default config for maximum retention days?

[wu-sheng]

This is log4j mechanism, out of SkyWalking scope. We don't change anything from standard log framework.