C API for checking health of network thread

It might be useful to have a function that returns a `Future*` that is completed by the network thread immediately.

@alecgrieser thinks that opening a transaction and committing it without doing anything else might be close in the current API.

Start by reading the current API involving Future* and the network thread, then compare it with the suggested empty transaction-and-commit flow. Determine how a function could return a Future completed immediately by the network thread, and define completion using the existing API behavior.

Written by the indexing model from the issue text.