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

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

Spring Integration 3.0 introduced registration `MessageSource` as beans
with `id` based on the adapter `id` or as fallback to generation from
`MessageSource` class name and the suffix `.source`.
But as far as that bean wasn't registered as bean with generated name (because suffix),
usage 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 using for fallback generation a `SourcePollingChannelAdapterFactoryBean` class name.