Add API for probing network thread at a priority

This is related to #4050.

We could add a function like `fdb_database_probe_main_thread(int priority)` that would let us schedule a task on the network thread to be executed at a given priority. Clients could time the response to get a sense for network thread latency at specific priorities. We could also provide some named priorities for ease of use (e.g. GRV_PRIORITY).

Right now there are a few functions being used to perform a similar task (e.g. GRV for a transaction that already has a read version, reading a key that is in RYW cache, read-only commit), but in these cases the priorities used don't always correspond with the ones we're interested in.