exit the client when exiting cash
Guard against Groovy's exposure of private fields

According to http://jira.codehaus.org/browse/GROOVY-1875, Groovy is dumb
and exposes private fields. This means that someone can get access to the
semi-sensitive client/session information stored in the "concourse" variable
or they could execute private methods of the Concourse.Client class. This
would potentially cause undefined behaviour and could be a security leak
so we must check every command to make sure that said command does not
contain a banned sequence of characters before we execute the command
Add locking to remaining read methods from Limbo that are called directly
from the Engine so we don't get inconsistent results caused by the
Engine transporting writes while we are reading
use <= timestamp comparison in find() method for consistency with database and other Limbo historical reads
Add transportLock to prevent the appearance of dropped writes. This may or
may not increase write speed as well, but i need to do more testing to verify this
unit test for GettingStarted tutorial