fix: use dsn::tools::native_aio_provider as default value of FLAGS_aio_factory_name

[levy5307]

1. In previous implementation, there are some code to control the value FLAGS_aio_factory_name. But it is a little complexity which can lead to problems. So in this pull request, I remove these code and use dsn::tools::native_aio_provider as default value of FLAGS_aio_factory_name. You can config FLAGS_aio_factory_name as dsn::tools::sim_aio_provider if you want to use sim_aio_provider

2. fix the bug below:

➜  pegasus git:(master) ✗ ./run.sh shell
W2021-03-04 13:30:50.875 (1614835850875376939 16541) : overwrite default thread pool for task RPC_CM_QUERY_PARTITION_CONFIG_BY_INDEX from THREAD_POOL_META_SERVER to THREAD_POOL_DEFAULT
W2021-03-04 13:30:50.875 (1614835850875414106 16541) : overwrite default thread pool for task RPC_CM_QUERY_PARTITION_CONFIG_BY_INDEX_ACK from THREAD_POOL_META_SERVER to THREAD_POOL_DEFAULT
Pegasus Shell 2.1.SNAPSHOT
Type "help" for more information.
Type "Ctrl-D" or "Ctrl-C" to exit the shell.

cannot find factory '' with factory type provider
	the following 2 factories are registered:
		dsn::tools::native_aio_provider (type: provider)
		dsn::tools::sim_aio_provider (type: provider)
	Please specify the correct factory name in your tool_app or in configuration file
The config file is: /home/mi/workspace/pegasus/config-shell.ini.16534
The cluster name is: onebox
The cluster meta list is: 127.0.0.1:34601,127.0.0.1:34602,127.0.0.1:34603
>>>