Ensure that the calling network connection is still open (i.e. that the client
hasn't disconnected on us already). If there is no client connection, or if the client
connection is known to have disconnected, throw an error.
Don't call this from a situation where you may not be in a network operation, otherwise,you'll
get spurious errors.
java.io.IOException - if there is no open network channel
Determine if we should treat the "server" as available or not. If true, then this server
can be considered active and actions can be taken against it. If false, then actions should
not be taken.
This is particularly prevalent in systems where a "server" is differentiated by sub-units (such as with
HBase, where a "server" is really a region + network layer).
true if the server should be considered available, false otherwise.