Use rfind to locate the @ in connection string

[tomas789]

Our counterparty uses email addresses as usernames. The connection string looks something like

ssh://john.doe@example.com:johnspassword@sftp-transparency.entsoe.eu

The resolve function looks for the @ when parsing the URL using find function. That finds the first @ which means the username is john.doe instead of john.doe@example.com. Using rfind instead resolves this issue. I'm not sure if this is valid according to the URL standard. It should not break any existing existing functionality as the hostname seems to not allow @ sign.

Perhaps a better approach would be to rely on an external crate for this. The url crate comes to mind.

[jonhoo]

This change is 

[NobodyXu]

Looks like we need to bump MSRV

[NobodyXu]

Given that all CI runs except for these two which runs cargo +nightly -Zminimal-version failed, it's probably a bug in getrandom v0.1.7