Previously, a number of usages of @ConditionalOnMissingBean prevented
a bean that implements an auto-configured bean's "main" interface from
causing the auto-configuration of the bean to back off. This would
happen when @ConditionalOnMissingBean did not specify a type, the
@Bean method returned the bean's concrete type, and that concreate
type implements a "main" interface.

This commit updates such usages of @ConditionalOnMissingBean to
specify the "main" interface as the type of the bean that must be
missing. This will allow, for example, the auto-configured
MongoTemplate bean to back off when a MongoOperations bean is defined.