Ratelimit IMAP reconnections

[link2xt]

IMAP loop should have an emergency ratelimit for the case IDLE is constantly skipped to avoid busy looping in case of bugs such as unparseable responses. We need to limit iterations per second to something like 5 iterations per minute but with a generous burst size and disable it for bots.

[link2xt]

We can also limit the number of reconnections, to once per minute or so. Currently if an error such as #4939 occurs, i.e. parsing of IMAP response fails, drop the connection, so limiting the number of reconnections is sufficient and the limit can be much lower than the number of fetches.