waits an increasing amount of time before attempting a reconnect.
Wait is capped at 16s. Prevents many reconnect attemps in a short time period.
Fixes commit 05ad96a30c ("imap: improve reconnect stability") that
improved the reliability of the reconnect mechanism but did not
implement controls to prevent the triggering of too many reconnects
within a short period of time.
Fixes: 05ad96a30c ("imap: improve reconnect stability")
Signed-off-by: Koni Marti <koni.marti@gmail.com>