INT-3240: i-c-a-parser: Fix gen. MessageSource Id

JIRA: https://jira.springsource.org/browse/INT-3240

Spring Integration 3.0 introduced registration of `MessageSource`s as beans
with an `id` based on the adapter `id` or, as a fallback, generated from
the `MessageSource` class name and the suffix `.source`.
But since that bean wasn't registered as a bean with the generated name (because the suffix was added),
subsequent elements get the same source bean name. This meant that
several anonymous `inbound-channel-adapter` with the same type (`file:`, `ftp:`, `twitter:` etc.)
caused an issue, when the same `MessageSource` bean was used by several `SourcePollingChannelAdapter` beans.

Fix the issue for fallback name generation by using the `SourcePollingChannelAdapterFactoryBean` class name.
Since that is registered, the source will always get a unique name, regardless of whether an id is provided.