more db pool size is needed for br to create many databases

[Leavrth]

Enhancement

Currently, BR only has 16 db pools to create databases, and it takes about more than one hour to create 200K databases. However, if BR uses 100 db pools to create databases, it only takes about half an hour to finish create 200K databases.

tidb/br/pkg/restore/snap_client/client.go

Line 69 in c550aae

defaultDDLConcurrency = 16

[princejha95]

Why we are only thinking till 200K databases ? Would this number can change in future ? If yes, we again have to make similar changes, right ?