Delay initializing the TxRelay data structure for a peer until we receive
a version message from that peer. At that point we'll know whether it
will ever relay transactions. We don't initialize the m_tx_relay
data structure if:

- this is an outbound block-relay-only connection; or
- fRelay=false and we're not offering NODE_BLOOM to this peer
  (NODE_BLOOM means that the peer may turn on tx relay later)