Split MIN_PROTO_VERSION into INIT_PROTO_VERSION and MIN_PEER_PROTO_VERSION

INIT_PROTO_VERSION is the initial version, after a succesful version/verack it is increased to a negotiated version.
MIN_PEER_PROTO_VERSION could be a different value to disconnect from peers older than a specified version.

Conflicts:
	src/main.cpp
	src/net.h

Conflicts:
	src/net.h
	src/version.h
Added ping time measurement.
New RPC "ping" command to request ping.
Implemented "pong" message handler.
New "pingtime" field in getpeerinfo, to provide results to user.
New "pingwait" field, to show pings still in flight, to better see newly lagging peers.

Conflicts:
	src/bitcoinrpc.cpp
	src/bitcoinrpc.h
	src/main.cpp
	src/net.cpp
	src/net.h
	src/rpcnet.cpp

Conflicts:
	src/main.cpp
	src/net.h
Ping automatically every 2 minutes (unconditionally)

... instead of after 30 minutes of no sending, for latency measurement
and keep-alive. Also, disconnect if no reply arrives within 20 minutes,
instead of 90 of inactivity (for peers supporting the 'pong' message).

Conflicts:
	src/net.cpp
	src/net.h
Use pong receive time rather than processing time

Conflicts:
	src/main.cpp
committing previous net code