Allow minWireVersion/maxWireVersion fields in heartbeat responses to be configured

[divjotarora]

mongobetween currently responds to heartbeats (periodic isMaster requests) with minWireVersion=0 and maxWireVersion=7 in order to present as a 4.0 server. To be more flexible, users should be able to configure these via command line parameters and/or environmental variables. This would allow mongobetween to support multiple server versions and would also make server upgrades easier because no code changes would be required to the proxy.