Full support of Ebean DataSource property names and bugfix

Extra property name "propertyNames" controls the behavior:

hikari-only - only Hikari properties will be used
ebean-only - only Ebean properties will be used
hikari-then-ebean - all properties will be loaded; Ebean (second) will override Hikari
ebean-then-hikari - (default) all properties will be loaded; Hikari (second) will override Ebean

datasource.db.propertyNames = hikari-only

appendFrom property to append settings from other db configurations
datasource.db2.appendFrom = db, mixin