Support RelayTx param in ver message for 70001+.

This commit adds support for the version message RelayTx parameter for
protocol version 70001+ as added by BIP0037.
Cleanup and finish relay transaction work.

- Coalesce the new bytes into the max message size constant to stay
  consistent
- Correct optional relay tx field handling
- Rename the relay transactions field to DisableRelayTx so the zero value
  of false has the correct default behavior
- Add tests for new bool fast paths in read/writeElement
- Stay consistent with version order in tests
- Add a single entry to TestVersionWire to test the new functionality
  instead of adding a whole new TextVersionRelayTx function.
- Use BIP0037 in tests instead of hard coding 70001
- Nuke XXX that 70001 is different since this is handled now
- Fix and cleanup some comments
- Update test coverage report