easy:Upkeep()
Currently the only protocol with a connection upkeep mechanism is HTTP/2: when the connection upkeep interval is exceeded and easy:Upkeep() is called, an HTTP/2 PING frame is sent on the connection.
This function must be explicitly called in order to perform the upkeep work. The connection upkeep interval is set
with #CURLOPT_UPKEEP_INTERVAL_MS
.