import: enable auto compactions in import mode (#4086)
import: enable auto compactions in import mode (#4086)

Disabling auto compactions in import mode leads to some problems:
1. a lot of files will be accumulated in level 0, which slows down the
   sst ingestion
2. if a tikv restarts accidentally, it will stall for a long time
3. if we forget to switch back to normal mode after importing, tikv
   will perform very bad

Enabling auto compactions also simplifies the import process because
we don't need to compact every batch. According to previous tests,
compactions are not the bottleneck during importing data with
tidb-lightning.

All in all, I think disabling auto compactions in import mode does
more harm than good.

Signed-off-by: Huachao Huang <f173607aa831848a3ee0de3a5ae0c89c11a68b51@gmail.com>
c4pt0r/tikv,pingcap/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv,pingcap/tikv