Add support for routing all storage operations through raft.

The tests pass with this change most of the time but there are some
timing issues around the initial election and tests using IsTrueWithin,
so everything still uses noopRaft until that can be fixed.

@spencerkimball @cockroachdb/developers 
Also, is it really conceivable that raft == nil? I'd rather just let this throw a nil pointer panic if it should never happen. Having it ignore the proposal seems pretty unfortunate.