CDRIVER-4130 Unconditionally use OP_MSG for server 5.1 and newer (#912)

* Warn and ignore exhaust cursor option given server 5.1 or newer
* Unconditionally use OP_MSG for find command given server 5.1 or newer
* Unconditionally use OP_MSG for getMore command given server 5.1 or newer
* Avoid unnecessary exhaust cursor in /counters/op_msg test
* Document exhaust option behavior for find command
* Add /Client/exhaust_cursor/fallback test for server 5.1 and newer