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.
Merge pull request #3155 from wtogami/split_proto_var

Split MIN_PROTO_VERSION into INIT_PROTO_VERSION and MIN_PEER_PROTO_VERSION