This PR is mainly do three things:
1. Fix fe meta version bug introduced by #4029 , when fix conflict with #4086
2. Make drop check code easy to read
3. Add doc content for drop meta check
[Bug]Fix some schema change not work right
This CL mainly fix some schema change to varchar type not work right
because forget to logic check && Add ConvertTypeResolver to add
supported convert type in order to avoid forget logic check
Currently we choose BE random without check disk is available,
the create table will failed until create tablet task is sent to BE
and BE will check is there has available capacity to create tablet.
So check backend disk available by storage medium will reduce unnecessary RPC call.
