Opening data dirs parallelly is not thread-safe

[vagetablechicken]

Describe the bug
162b1c5 makes the data dirs init parallelly, but I found it's not thread-safe.
When be is starting, it may get this error:

[data_dir.cpp:249] fail to find file system, path=/xxx

I didn't notice that DataDir::_init_file_system() may have race conditions.
getmntent() value is MT-Unsafe race:mntentbuf locale
so mount_entry may be the wrong value NULL , is_find will be false.
https://github.com/apache/incubator-doris/blob/614a76beeac73821c78903c46e7a703b7956796b/be/src/olap/data_dir.cpp#L228-L251

We have two solutions:

1. use getmntent_r() instread, getmntent_r() value is MT-Safe locale.
2. split _init_meta()(open rocksdb), just _init_meta() in parallel. Keep other parts of data dir init in sequential order.

[imay]

I prefer option 1, it seems simple. Because the master branch has bug, hope that you can fix it ASAP.

[vagetablechicken]

I prefer option 1, it seems simple. Because the master branch has bug, hope that you can fix it ASAP.

OK, it is my primary task today.