Support namespace mappings in MultiStorage

[brfrn169]

Currently, we only have a table mapping support in MultiStorage as follows:

# Define table mappings from table names to storages. The format is “<table name>:<storage name>,...”
scalar.db.multi_storage.table_mapping=user.ORDER:cassandra,user.CUSTOMER:mysql,coordinator.state:cassandra

This PR adds a namespace mapping support where we can define namespace mappings as follows:

# Define namespace mappings from namespace names to storages. The format is “<namespace name>:<storage name>,...”
scalar.db.multi_storage.namespace_mapping=user:cassandra,coordinator:mysql

Please take a look!

[feeblefakie]

Overall looking good. Thank you!
Left a minor question and comments.

[feeblefakie]

It is a minor style thing, but I feel the following is slightly better since less indenting.

if (tableMapping == null) {
  return null;
}
for (String tableAndStorage : tableMapping.split(",")) {
  // ...
}

[feeblefakie]

Ditto.

[feeblefakie]

So, if conflicting mapping like below is given, it prefers namespace mapping, right? Or we shouldn't use both mapping format together?

scalar.db.multi_storage.table_mapping=user.ORDER:cassandra,user.CUSTOMER:mysql,coordinator.state:cassandra
scalar.db.multi_storage.namespace_mapping=user:cassandra

Anyway, I think it would be great if we note it somewhere.

[brfrn169]

Thank you for reviewing this!

If conflicting mapping between a table mapping and a namespace mapping, it prefers the table mapping because table mappings are more specific than namespace mappings.

Sure, I will add it to the Javadoc. Thanks.

[feeblefakie]

Ah, sorry, I meant table mapping is preferred. Thank you!

[feeblefakie]

LGTM! Thanks!
I left minor suggestions on the English comments.

[feeblefakie]

Ah, sorry, I meant table mapping is preferred. Thank you!

[feeblefakie]

LGMT!