The `_request` field on an AsyncOp represents an overarching user request being run through NetworkInterfaceASIO. We may call _asyncRunCommand many times within that one user request. The underlying _request field should not change for each "mini request" run through _asyncRunCommand. By accessing this field within that method we will log incorrect/misleading things about what command we are running.

For example, when we refresh connections, we re-use part of the NetworkInterfaceASIO state machine. When we are refreshing connections, we do not set the _request field, which leads to confusing log messages with default information:

"Failed to heartbeat to :27017 - HostUnreachable: An established connection was aborted by the software in your host machine."

Note that this log line is from a cluster that did not have a member running on localhost:27017, and had never before connected to a host at :27017.

This extends to any code that we might flow through from _asyncRunCommand before returning from running the command, such as _validateAndRun and associated paths.