Peer preferentially with NODE_CASH nodes

Summary:
Add NODE_BITCOIN_CASH to required services for outbound
peer connections.
Not required for inbound peers.

Test Plan:

• Remove peers.dat
• Start up node
• Monitor peer connections using e.g. $ bitcoin-cli getpeerinfo | egrep '(service|subver|inbound)'
• Check that all peers for which inbound is false have bit 5 (0x20) set in their reported services
• Connect to various other types of peers (BU, Classic, XT) using addnode and check that connections are successful
• Check that inbound peers without NODE_CASH service bit set can connect (you should get occasional crawlers or Satoshi clients connecting with services < 0x20)

Reviewers: #bitcoin_abc, deadalnix, CCulianu

Reviewed By: #bitcoin_abc, CCulianu

Subscribers: CCulianu

Differential Revision: https://reviews.bitcoinabc.org/D457