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