Drop table causes be crash

[HangyuanLiu]

Describe the bug
Drop table causes be crash after schema change

To Reproduce
Steps to reproduce the behavior:

1. schema change
2. drop table
3. create table again

error log :
F0507 10:25:01.622939 19737 utils.cpp:1098] fail to destroy mutex. err=Device or resource busy

[kangpinghuang]

hi, the reason of this problem is the schema change lock is not released when drop tablet.
This is a bug of BE, it will be solved in release 0.10 soon.