Add match checking for orderbook after xud connection establishing

[raladev]

For now we have following situation:

1. Alice has buy 1 BTC/LTC 10 order in order book
2. Alice node know nothing about Bob's node
3. Bob has sell 1 BTC/LTC 10 order in order book
4. Alice is connecting to Bob's xud

Actual result: No order matching, 2 orders exists in orderbooks of both users

Expected result: After 4 step orderbooks should be checked for crossed orders and orders should be matched. Alice is taker, Bob is Maker.

[kilrau]

Reason for this being in this repo: we should add some basic guidelines on order matching to the specs