Connect to only one coin electrum at the time

[tonymorony]

Noticed that atomicDEX-API keeping connections to all electrums from electrum RPC call params.

Since connection operation is very fast I think it might be worth to keep a connection to single electrum only and connect to another only in case of connectivity issues to the active one.
It'll reduce the number of connections to electrums multiple times (what can reduce the number of file descriptors what might allow activating more coins on mobile lets say)

[artemii235]

Thanks for opening the issue! It is a nice enhancement indeed. We can mix it with the general Electrum client refactoring: #746. I guess we will find the time to implement this next month.

[shamardy]

Fixed in #1966