Reject multiple sessions with the same host key

Reject multiple sessions with the same host key

SSH host key is supposed to be unique among devices. If there is an
attempt to establish a session with the same key, terminate the new
session.

Since that rules out the possibility of multiple remotes for a key,
refactor the code to use a ConcurrentMap instead of a Multimap --
removing the need for any locking.

As we really want to use CallHomeProtocolSessionContext in logging,
make it implement a toString() method and clean it up a bit.

JIRA: NETCONF-653
Change-Id: I2d78b9aa34ee4b16d0c6d2ed2ac172942c575851
Signed-off-by: Manoj Chokka <manoj.chokka@verizon.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>