Add new defvoo nnimap-keepalive-intervals to Gnus nnimap servers
* lisp/gnus/nnimap.el (nnimap-keepalive-intervals): New per-server config for customizing when keepalive commands are sent. (nnimap-keepalive, nnimap-open-connection-1): Consult in these places. Additionally, use nnimap-streaming -> t when sending the keepalive NOOP, so we don't wait for the response. * doc/misc/gnus.texi (Customizing the IMAP Connection): Document.