*: skip creating Domain for backup without stats

[overvenus]

What problem does this PR solve?

This PR reduces memory usage during backup by skipping initializing Domain which loads all table info into memory.

Close #818

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Manual tests

1. Load 40k 40 fields YCSB tables

for i in $(seq 1 40000); do { go-ycsb load mysql -p workload=core -p mysql.host=172.16.5.33 -p mysql.port=47904 -p mysql.user=root -p recordcount=20 -p fieldcount=40 -p fieldlength=16 -p threadcount=4 -p table=usertable$i; } done;

2. Backup 40k tables

br --pd http://PD_ADDR backup full -s "local:///path/to/backupdir"

The master branch BR takes about 3.09GB of memory.

This PR BR takes about 2.21G of memory.

Code changes

• Has exported function/method change
• Has exported variable/fields change

Related changes

• Need to cherry-pick to the release branch

Release Note

• Reduce memory usage during backup.

[overvenus]

/run-all-tests

[overvenus]

/run-all-tests

[kennytm]

rest LGTM

[YuJuncen]

/lgtm

[kennytm]

/lgtm

[ti-srebot]

/run-all-tests

[ti-srebot]

cherry pick to release-4.0 in PR #886

[ti-srebot]

cherry pick to release-5.0 in PR #887