Consolidate ChainEndpoint::proven_*() functions with their analogous query_*() version

[plafer]

I just noticed that the ChainEndpoint::proven_*() methods each have an analogous query_*() method which performs the same query except that they don't return a proof.

I suggest we remove the proven_*() methods from ChainEndpoint and ChainHandle, and change the provable query_*() functions as follows:

// previous
fn query_channel(&self, request: QueryChannelRequest) -> Result<ChannelEnd, Error>;
fn proven_channel(&self, request: QueryChannelRequest) -> Result<(ChannelEnd, MerkleProof), Error>;

// new
fn query_channel(&self, request: QueryChannelRequest, include_proof: PerformQuery) -> Result<(ChannelEnd, Option<MerkleProof>), Error>;

enum PerformQuery {
    WithProof,
    WithoutProof,
};

I introduced PerformQuery to avoid boolean blindness.

Example calls:

chain.query_channel(request, PerformQuery::WithProof);
chain.query_channel(request, PerformQuery::WithoutProof);

An alternative name for PerformQuery I thought of:

enum IncludeProof {
    Yes,
    No,
};
chain.query_channel(request, IncludeProof::Yes);
chain.query_channel(request, IncludeProof::No);

I personally prefer merging the two functions into one, as we expose fewer methods while preserving the same functionality.

Originally posted by @plafer in #2200 (comment)