Replacement for elements of #96 more specific to specification. These need to be updated synchronously with the spec:

• capture the validation functions more explicitly in the spec and align with their use in the code
• redundant check for sigs (Light client: further trait impls, improvements, and tests #84 (comment), Update specification of light client algorithm to align with the code tendermint/spec#61 (comment))
• Improve naming of variables like h1_xxx and h2 (Light client: bisection, store, requester #100 (comment), Light client: bisection, store, requester #100 (comment))
• how to get latest height from a node? use height=0 ? Could get rid of get_smaller_or_equal (Light client: bisection, store, requester #100 (comment))
• improve logic of check_support to better clarify the 3 conditions: ok (update state), real error (faulty node or system), not-enough-voting-power (not real error, just need to bisect)
• possible move validity checks from check_support to valdiate_vals_and_commit ...
• add a single function that can be called by IBC that doesn't use a requester but otherwise does all checks necessary to update the store with a new trusted state. Ideally this function can be used by a function that does use the Requester