Previously, Spring Boot mapped both `DataSourceProperties` and the actual
`DataSource` implementation to the same prefix. This results in a huge
amount of keys in the `spring.datasource` namespace  with no way to
identify those that are valid for the pooled data source in use.

This commit maps the four pooled data sources we support in four isolated
namespace, keeping `spring.datasource` only for the common settings.

These are `spring.datasource.tomcat`, `spring.datasource.hikari`,
`spring.datasource.dbcp` and `spring.datasource.dbcp2` for the Tomcat,
Hikari, Commons DBCP and Commons DBCP2 implementations respectively.

Closes gh-2183