INT-3147: (3167,3173,1941) Improve MetadataStore

Previously, `MetadataStore` couldn't be configured for Twitter Adapters
- only global one.
The `metadataKey` was generated automatically with 'difficult' value

* Register all `MessageSource` for `SourcePollingChannelAdapter`
as beans with id based on adapter id and prefix '.source' (INT-3147)
* Polishing parser to get rid of explicit `MessageSource` beans. (INT-3147)
* Make Feed and Twitter adapters `id` attribute as required -
now it presents a `metadataKey` for `MetadataStore` (INT-3147)
* Add to Twitter adapters a reference attribute for `MetadataStore` (INT-3173)
* Add Twitter adapters `poll-skip-period` attribute (INT-3167)
* Add and implement `MetadataStore#remove` (INT-1941)
* Make `MetadataStore` as `@ManagedResource` (INT-1941)
* Polishing tests

JIRAs:
https://jira.springsource.org/browse/INT-3147
https://jira.springsource.org/browse/INT-3167
https://jira.springsource.org/browse/INT-3173
https://jira.springsource.org/browse/INT-1941