We still prefer Tomcat if it is available (that can change
if the community asks loudly enough). Hikari is supported
via the same spring.datasource.* properties as Tomcat (and
DBCP), with some modifications:

* The validation and timeout settings are not as fine-grained
in Hikari, so many of them will simply be ignored. The most
common options (url, username, password, driverClassName) all
work as expected.

* The Hikari team recommends using a vendor-specific DataSource
via spring.datasource.dataSourceClassName and supplying it with
Properties (spring.datasource.hikari.*).

Hikari prefers the JDBC4 isValid() API (encapsulates vendor-
specific queries) which is probably a good thing, but we
haven't provided any explicit support or testing for that yet.

Fixes gh-418