Refactor to_send_clients to use a BTreeMap
Refactor to_send_clients to use a BTreeMap

This is both a performance optimization (avoiding O(n) shifting from the
beginning), and communicates intent in a nicer way overall.

It is plausible that we will eventually want to tie this data structure to
something like the DependencyQueue, i.e., to get more information on which rustc
to give tokens to. An old rustc with a very late dependency edge is less
important than one we'll need sooner, probably.

vojtechkral/cargo,rust-lang/cargo,tshepang/cargo,vojtechkral/cargo,eminence/cargo,nipunn1313/cargo,tshepang/cargo,rust-lang/cargo,vojtechkral/cargo,rustyhorde/cargo,rust-lang/cargo,tshepang/cargo,tsurai/cargo,nipunn1313/cargo,tsurai/cargo,tsurai/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,vojtechkral/cargo,eminence/cargo,tsurai/cargo,nipunn1313/cargo,rustyhorde/cargo,tsurai/cargo,rust-lang/cargo,eminence/cargo,rustyhorde/cargo,tshepang/cargo