Rename blocking Camel methods.

Rename blocking Camel methods.

Rename blocking Camel methods by appending a "sync" suffix.  e.g.
camel_folder_get_message() becomes camel_folder_get_message_sync().

Several reasons:

The asynchronous functions which are about to be added will take the old
names, along with a corresponding "finish" function.

Along with the GCancellable argument, the "sync" suffix makes it clear
to clients that the function may block.  Clients are encouraged to use
the asynchronous functions instead, unless they know for certain the
function will not block (e.g. writing to a CamelMemStream).

The "sync" suffix makes it easy to grep for blocking Camel calls in
Evolution and identify places where the main loop is being blocked.
These are bugs, and they're still lurking in Evolution.