Use com.sun.mail:javax.mail for ReadableMime dependency

com.sun.mail:javax.mail is already part of the Spring IO Platform,
whereas com.sun.mail:mailapi is not.
INT-3501 Fix IMAP Idle

JIRA: https://jira.spring.io/browse/INT-3501

There were several problems:

When a `Folder` is open, an activity on the `Store` opens a new
connection.

The `PingTask` called `isConnected()` on the store, creating a new
connection. The pings (NOOPs)  were performed on this connection and
therefore did NOT cancel the `IDLE`.

In any case, the `IDLE` was issued on the folder not the store and
the only way to cancel that IDLE is to cause its `waitIfIdle()` to
be invoked. Conveniently, simply calling `isOpen()` invokes that
method and cancels the IDLE.

Finally, the `SimpleMessageCountListener` unnecessarily invoked
`message.getLineCount()` when a simple `folder.isOpen()` is
sufficient.

Fixes:

1. Do not invoke `openSession` if `this.folder` is not null.
2. Change the `PingTask` to simply invoke `isOpen()`.
3. Move the `PingTask to the receiver for a more efficient algorithm
    instead of running on fixed interval. Rename it `IdleCanceler`.
4. Increase the PING timer from 10 to 120 seconds; add setters for it
    and the `reconnectionDelay`.

TODO: Namespace support for 4.1. (Not to be back ported).

Add a test IMAP server (ported from Java DSL and enhanced to simulate IDLE
with a new message arriving for the first idle period.

INT-3501 Polishing - PR Comments
INT-3504: Deprecate `AmqpHeaders`

JIRA: https://jira.spring.io/browse/INT-3504

* Deprecate `AmqpHeaders` and extend it from `org.springframework.amqp.support.AmqpHeaders` for backward compatibility
* Change AMQP dependency to `1.4.0`
* Remove usage of deprecated `spring_reply_correlation` and `spring_reply_to` AMQP headers
* Fix syslog `DefaultMessageConverter` JavaDoc

Reformat code
INT-3507 TCP - Use BufferedOutputStreams

JIRA: https://jira.spring.io/browse/INT-3507

TcpNoDelay (false) helps to buffer IOs but only after the
first write.

Use `BufferedOutputStreams` for TCP writes.

Use the socket `sendBufferSize` for the buffer size.
INT-3506 Async Gateway Improvements

JIRA: https://jira.spring.io/browse/INT-3506
JIRA: https://jira.spring.io/browse/INT-3428

Support flows downstream of the gateway that support
returning a `Future<?>` payload.

Currently, any method that returns a type that is assignable
to `Future<?>` runs async and returns a `FutureTask<?>`.

This prevents a service-interface method that returns a
custom `Future<?>` object from being invoked without wrapping
that `Future<?>` in a `FutureTask<?>`.

Allow the async-executor to be set to `null` causing any method
returning `Future<?>` to run on the calling thread.

Add support for `ListenableFuture<?>`.

If the return type is a `RunnableFuture`, `ListenableFuture` or
`Future`, or exactly a `FutureTask` or `ListenableFutureTask`, run the flow
on the executor (if present); otherwise run on the calling thread.

INT-3506 Fix Object returnType

INT-3506 Polishing - PR Comments

Perform dummy invocations of `submit` and `submitListenable` to
determine the actual return types so that we can determine at
runtime whether the executor will return a type that is compatible
with the method return type. If not, run on the caller's thread.

Add DEBUG Log If Incompatible Future<?>

INT-3506 Add Support for MessagingGateway

Add a constant to indicate no executor.

Add tests.

INT-3506 Polishing and Docs

- Docbook
- XSD
- Change test to send calling thread in payload so we can determine whether
   we need to return a Future or not; previously relied on the thread name
   which was brittle.

Polishing JavaDocs.
Change `amqp.xml` to use `org.springframework.amqp.support.AmqpHeaders` instead of an old one.