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.
6 jobs for master in 10 minutes and 8 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Fast
failed #18902
test-fast-inotify

00:07:35

 
  Normal
failed #18903
test-lisp-inotify

00:02:32

passed #18904
test-lisp-net-inotify

00:01:52

 
  Slow
skipped #18905
build-native-bootstrap-speed0
skipped #18906
build-native-bootstrap-speed1
skipped #18907
build-native-bootstrap-speed2
 
Name Stage Failure
failed
test-lisp-inotify Normal
make: *** [check-lisp] Error 2
make: Leaving directory '/checkout/test'
Running after_script
Running after script...
$ test -n "$(docker ps -aq -f name=${test_name})" && docker cp ${test_name}:checkout/test ${test_name}
$ test -n "$(docker ps -aq -f name=${test_name})" && docker rm ${test_name}
test-lisp-inotify-aeada12e
Cleaning up file based variables
ERROR: Job failed: exit code 2
failed
test-fast-inotify Fast
Makefile:330: recipe for target 'check-doit' failed
make[1]: Leaving directory '/checkout/test'
Makefile:300: recipe for target 'check' failed
make: Leaving directory '/checkout/test'
Running after script...
$ test -n "$(docker ps -aq -f name=${test_name})" && docker cp ${test_name}:checkout/test ${test_name}
$ test -n "$(docker ps -aq -f name=${test_name})" && docker rm ${test_name}
test-fast-inotify-aeada12e
ERROR: Job failed: exit code 2