Use NETINFO handshake rather than date header to check time with authorities

tor currently checks its clock against the directory authorities by reading the HTTP date header in the directory documents.

In legacy/trac#15775 (moved), we allow clients to bootstrap using fallback directories, rather than authorities.

In legacy/trac#4483 (moved), we make multiple connections, and use the first connection that starts downloading. If there are multiple connections downloading, we favour authority connections, so that tor can still get a clock check.

But if tor used the date from the TLS handshake, it could get directory documents from a fallback directory, and abort authority connections sooner. This would place less load on the authorities.

This would be similar to the tlsdate implementation: https://github.com/ioerror/tlsdate

Edited: Look at the netinfo cell, not the TLS handshake. -- nickm