The default values of shard_row_id_bits and pre_split_regions should be configurable

[jackysp]

Feature Request

Is your feature request related to a problem? Please describe:

The default values of shard_row_id_bits and pre_split_regions are 0, which make the customers have to set it one by one on their table schemas.

Describe the feature you'd like:

Make the default values of shard_row_id_bits and pre_split_regions be configurable.

Describe alternatives you've considered:

No.

Teachability, Documentation, Adoption, Migration Strategy:

When a new customer want to migrate his database to TiDB, we nearly always suggest them to set shard_row_id_bits and pre_split_regions on their tables, make the default values configurable will make the migration easier.

[jackysp]

Related to https://internal.pingcap.net/jira/browse/FRM-22