Added "SSH Host key verification" logic in SSH Tunneling code

Hi Dave

As per your suggestion I have implemented the "SSH Host key verification"
logic in SSH Tunneling code. Below is the brief description about feature:

When connecting to an SSH server for the first time, the user should be
presented with a prompt showing the host key, and given the option to
accept or reject it. If accepted, the key should be cached and the
connection should proceed. If rejected, the connection should be
immediately aborted.

When connecting on subsequent occasions, pgadmin should check the host key
against the cached copy. If they match, the connection should proceed as
normal. If they do not match, the user should be presented with a prominent
warning showing them both the expected and received host keys, and giving
them the option to reject (the default) or accept the new key. If reject is
chosen, the connection should be immediately aborted and the cached key
should not be updated. If accepted, the connection should proceed and the
cached key should be updated with the new one.

Attached is the patch file, can you please review it. If it looks good to
you then can you please commit it.

--
*Akshay Joshi
Senior Software Engineer
EnterpriseDB Corporation
The Enterprise PostgreSQL Company
Phone: +91 20-3058-9522
Mobile: +91 976-788-8246*