evidence: protect valToLastHeight w/ mtx
evidence: protect valToLastHeight w/ mtx

to prevent data races

note: when we check the ConflictingHeadersEvidence, the copy is used.

Closes #4749

Also, iterate over valToLastHeight instead of loading validators. While it might be slower than accessing a single key in goleveldb, the new code is better adapted to ConsensusParams changes.
tendermint/tendermint,tendermint/tendermint,tendermint/tendermint,tendermint/tendermint