[IMAPx] Enable concurrent connections

This returns back CamelIMAPXConnManager, and enables use of concurrent
connections with it, which helps with response time of user requests.
[IMAPx] Do not open each folder on LIST/LSUB responses

It has sever performance issues, especially with large folders,
which load (part of) the folder summary into memory too.
Bug #722275 - IMAPX: Incorrectly reads astring values (mailbox names)
Always reject revoked certificates

If there is recognized a revoked certificate being used for a secure
connection, then reject the connection immediately, for security reasons.
This behaviour cannot be overwritten with a user's trust.
camel-imapx-conn-manager.h/c: Correct LGPL license notices
NEWS update for 3.12.1 release.
Post-release version bump.
Updated Spanish translation
[IMAPX] Do not expose user credentials in debug logs

And skip trailing \r\n in the io logs as well.
Bug #727598 - [IMAPX] Fails to list folders without NAMESPACE capability
Set properly children availability for newly created local folders

This is to help with a UI regression mentioned at
https://bugzilla.gnome.org/show_bug.cgi?id=722698#c13
with the local folders.
[IMAPx] Automatically limit maximum number of concurrent connections

In case a user has set more connections than the server has allowed,
the IMAPx may limit the connection count automatically, instead of
panic and close already established connections.