nacos server can not start on personal/test computer with small/low memory

[pguo65535]

Description

the default start JVM heap size 2g is too large for personal testing purpose.

JAVA_OPT="${JAVA_OPT} -server -Xms2g -Xmx2g -Xmn1g -XX:MetaspaceSize=128m -XX:MaxMetaspaceSize=320m"

[pguo65535]

I think we can optimize the startup script， when user are running Nacos in standalone mode（means in personal study or testing environment) we can adjust the GC policy to the JVM default policy and set a smaller heap size and

but for production mode(cluster mode), we can recommend a GC policy to get a low latency and GC time spend

[pguo65535]

Josh's issue #214 are also related to this point