Make the netmgr read callback to be asynchronous only when needed

Previously, the read callback would be asynchronous only on success or timeout. Add an option (similar to what other callbacks have) to decide whether we need the asynchronous read callback on a higher level.

On a general level, we need the asynchronous callbacks to happen only when we are invoking the callback from the public API. If the path to the callback went through the libuv callback or netmgr callback, we are already on asynchronous path, and there's no need to make the call to the callback asynchronous again.

Closes #3693 (closed), #3696 (closed)