Introduce whitelisted peers.

This adds a -whitelist option to specify subnet ranges from which peers
that connect are whitelisted. In addition, there is a -whitebind option
which works like -bind, except peers connecting to it are also
whitelisted (allowing a separate listen port for trusted connections).

Being whitelisted has two effects (for now):
* They are immune to DoS disconnection/banning.
* Transactions they broadcast (which are valid) are always relayed,
  even if they were already in the mempool. This means that a node
  can function as a gateway for a local network, and that rebroadcasts
  from the local network will work as expected.

Whitelisting replaces the magic exemption localhost had for DoS
disconnection (local addresses are still never banned, though), which
implied hidden service connects (from a localhost Tor node) were
incorrectly immune to DoS disconnection as well. This old
behaviour is removed for that reason, but can be restored using
-whitelist=127.0.0.1 or -whitelist=::1 can be specified. -whitebind
is safer to use in case non-trusted localhost connections are expected
(like hidden services).
Merge pull request #4378

dc942e6 Introduce whitelisted peers. (Pieter Wuille)