Before we were storing trustedHeader (height=1) and trustedNextVals
(height=2).
After this change, we will be storing trustedHeader (height=1) and
trustedVals (height=1). This a) simplifies the code b) fixes#4399
inconsistent pairing issue c) gives a relayer access to the current
validator set #4470.
The only downside is more jumps during bisection. If validator set
changes between trustedHeader and the next header (by 2/3 or more), the
light client will be forced to download the next header and check that
2/3+ signed the transition. But we don't expect validator set change too
much and too often, so it's an acceptable compromise.
Closes#4470 and #4399
* adr: light client implementation
Closes#2133
* note on chain IDs
* explain why witnesses are required
* if chain forks maliciously, chain ID stays the same
* add a note about min witnesses while cross-checking
