BUG/MAJOR: sessions: Use an unlimited number of servers for the conn list.

BUG/MAJOR: sessions: Use an unlimited number of servers for the conn list.

When a session adds a connection to its connection list, we used to remove
connections for an another server if there were not enough room for our
server. This can't work, because those lists are now the list of connections
we're responsible for, not just the idle connections.
To fix this, allow for an unlimited number of servers, instead of using
an array, we're now using a linked list.