When BE sets `ignore_broken_disk` to true, it's expected that non-exist path in storage_root_path won't prevent BE from launching, but in 0.12 BE fails to launch in such scenario.
```
W0506 14:46:11.039953 17040 options.cpp:64] path can not be canonicalized. may be not exist. path=/data11/olap
W0506 14:46:11.040014 17040 options.cpp:141] failed to parse store path /data11/olap, res=-203
```
The reason is that #2861 adds a path existence check in `parse_root_path` which precedes the usage of `ignore_broken_disk` in the main method.
* Add streaming load feature. You can execute 'help stream load;' to see more information.
Changed:
* Loading phase of a certain table can be parallelized, to reduce the load job execution time when multi load jobs to a single table.
* Using RocksDB to save the header info of tablets in Backends, to reduce the IO operations and increate speeding of restarting.
Fixed:
* A lot of bugs fixed.
