Java DSL: Lambda_Factories

* Change all `IntegrationFlows` methods to `.from()`
* Introduce internal `Function<?, ?>` extensions for particular cases, e.g. `MessageSourcesFunction`.
Now end-user has to cast Lambda parameter to concrete type to get desired factory, e.g.:
```
return IntegrationFlows.from((MessageProducers mp) -> mp.imap("imap://user:pw@host/INBOX"))
```
* rename `.fromFixedMessageChannel` to `.from()` with additional boolean flag
* Introduce `PollerFactory` Lambda for `.poller()` EIP-method on `EndpointSpec`
* Move `Pollers` stuff to the `.core` package to fix package tangle
* Refactoring for some code style
* Fix some typos