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.

This commit alone does not change any behavior.  It stresses the difference between the initial and minimum version, the latter of which can be set to enforce a minimum version in a later policy discussion.