RUBY-831 Catch OperationTimeout in MongoClient#active? and return false

The user who opened ticket RUBY-831 wanted a way to determine if a server was reachable through the #active? method on a client. I decided that I didn't want to mix the semantics of #active? and the concept of a server being reachable within the op_timeout number of seconds. I therefore added a method #accessible? on a client that would rescue a OperationTimeout and return false. This allows someone to separately determine if a connection can be made to a server and if an operation can be done in < op_timeout number of seconds.
Thank you for working on my request. I think this one though will not allow setting `:op_timeout` to nil as it was possible before. Also documentation states the default is to wait indefinitely.

I don't advocate waiting forever makes any sense. Actually I think that having a default timeout makes a lot of sense. I can't think of a practical purpose to have have an infinite timeout. But with my QE hat on, there might possibly be surprised existing users who expect to wait for a connection forever.
This means it has to be communicated in the docs as well maybe announce to users through a blog, mailing list or whatever.