IMAP server does not support multiple connections

[dvalter]

As @v01d mentioned in #23, IMAP server in Hydroxide does support only one active connection per account. Any but first connection does not progress to AUTHENTICATE completed usless previous one would be closed.

This is crucial (and worth documenting at very least) due to Thunderbird (and proboably some other clients) by default use multiple connections (5 in my case), which made hydroxide usage virtually impossible due to

I'm trying hydroxide with Thunderbird and I noticed that I can only access a folder, after which clicking on other folders does not work.

Steps to reproduce

0. (Optionnaly) start traffic recording with tcpdump/wireshark/whatever on loopback interface.
1. Open IMAP socket and login as <user>@protonmail.com
2. Repeat the same without closing previous connection via another instance of any client or simply telnet
3. It will stuck at AG client request

Fix for Thunderbird

1. Go to Account Setting (via Edit, or "Burger"->Preferences)
2. Click Server Settings for the Protonmail account
3. Click Advanced... button
4. Set Maximum number of server connections to cache to 1

I hope this issue at least will help someone to sort out "Connection to server 127.0.0.1 timed out" errors in Thunderbird.