struct ddl doesn't have sessPool if RunWorker == false

[wjhuang2016]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

According to

	if RunWorker {
		d.sessPool = newSessionPool(ctxPool)
        }

struct ddl doesn't have sessPool if RunWorker == false.

There may cause some problems if ddl use sessPool later.
For example

1. the check checkDeleteRangeCnt would use it.
2. alter table Shard RowID would use it.

4. What is your TiDB version? (Required)

master