Add a property to configure the connection pool

A new `spring.datasource.type` property can now be used to configure the
connection pool implementation to use (rather than only relying on Boot's
preferences).

Closes gh-3705

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -318,6 +318,7 @@ content into your application; rather pick only the properties that you need.
 	spring.datasource.platform= # the platform to use in the schema resource (schema-${platform}.sql)
 	spring.datasource.continue-on-error=false # continue even if can't be initialized
 	spring.datasource.separator=; # statement separator in SQL initialization scripts
+	spring.datasource.type= # fully qualified name of the connection pool implementation to use
 	spring.datasource.driver-class-name= # JDBC Settings...
 	spring.datasource.url=
 	spring.datasource.username=

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -2047,8 +2047,10 @@ Here's the algorithm for choosing a specific implementation:
 If you use the `spring-boot-starter-jdbc` or `spring-boot-starter-data-jpa`
 '`starter POMs`' you will automatically get a dependency to `tomcat-jdbc`.
 
-NOTE: Additional connection pools can always be configured manually. If you define your
-own `DataSource` bean, auto-configuration will not occur.
+NOTE: You can bypass that algorithm completely and specify the connection pool to use via
+the `spring.datasource.type` property. Also, additional connection pools can always be
+configured manually. If you define your own `DataSource` bean, auto-configuration will
+not occur.
 
 DataSource configuration is controlled by external configuration properties in
 `+spring.datasource.*+`. For example, you might declare the following section in