net: Automatically create hidden service, listen on Tor

Starting with Tor version 0.2.7.1 it is possible, through Tor's control socket
API, to create and destroy 'ephemeral' hidden services programmatically.
https://stem.torproject.org/api/control.html#stem.control.Controller.create_ephemeral_hidden_service

This means that if Tor is running (and proper authorization is available),
bitcoin automatically creates a hidden service to listen on, without user
manual configuration. This will positively affect the number of available
.onion nodes.

- When the node is started, connect to Tor through control socket
- Send `ADD_ONION` command
- First time:
    - Make it create a hidden service key
    - Save the key in the data directory for later usage
- Make it redirect port 8333 to the local port 8333 (or whatever port we're listening on).
- Keep control socket connection open for as long node is running. The hidden service will
  (by default) automatically go away when the connection is closed.

src/Makefile.am

@@ -149,6 +149,7 @@ BITCOIN_CORE_H = \
   threadsafety.h \
   timedata.h \
   tinyformat.h \
+  torcontrol.h \
   txdb.h \
   txmempool.h \
   ui_interface.h \
@@ -206,6 +207,7 @@ libbitcoin_server_a_SOURCES = \
   rpcserver.cpp \
   script/sigcache.cpp \
   timedata.cpp \
+  torcontrol.cpp \
   txdb.cpp \
   txmempool.cpp \
   validationinterface.cpp \