Introduces heuristics that track the amount of no responses or unavailable blocks a provider has for more robust provider handling by the light client. Use concurrent calls to all witnesses when a new primary is needed.
## Description
I'm just doing a self audit of the light client. There's a few things I've changed
- Validate trust level in `VerifyNonAdjacent` function
- Make errNoWitnesses public (it's something people running software on top of a light client should be able to parse)
- Remove `ChainID` check of witnesses on start up. We do this already when we compare the first header with witnesses
- Remove `ChainID()` from provider interface
Closes: #4538
Callum WatersandGitHub
Anton KaliaevandGitHub
MarkoandGitHub