increase minimal bandwidth requirements, update the manpage, relay guide and FAQ

The man page has old minimal bandwidth requirements, let's update them to be consistent with the relay guide and the FAQ

man page currently says: https://www.torproject.org/docs/tor-manual-dev.html.en#BandwidthRate " If you want to run a relay in the public network, this needs to be at the very least 75 KBytes for a relay (that is, 600 kbits) or 50 KBytes for a bridge (400 kbits)  but of course, more is better; we recommend at least 250 KBytes (2 mbits) if possible. "

Proposed version for the man page: " If you want to run a relay in the public network, this needs to be at the very least 8 MBit/s (Mbps) for a relay or 1 MBit/s for a bridge   but of course, more is better; we recommend at least 16 MBit/s if possible. "

FAQ values: https://www.torproject.org/docs/faq.html.en#HowDoIDecide relay guide: https://trac.torproject.org/projects/tor/wiki/TorRelayGuide#BandwidthandConnections

context: https://lists.torproject.org/pipermail/tor-dev/2018-February/012916.html