Registering gRPC balancers is thread-unsafe because they are stored in a
global map variable that is accessed without holding a lock. Therefore,
it's expected that balancers are registered _once_ at the beginning of
your program (e.g. in a package `init` function) and certainly not after
you've started dialing connections, etc.
> NOTE: this function must only be called during initialization time
> (i.e. in an init() function), and is not thread-safe.
While this is fine for us in production, it's challenging for tests that
spin up multiple agents in-memory. We currently register a balancer per-
agent which holds agent-specific state that cannot safely be shared.
This commit introduces our own registry that _is_ thread-safe, and
implements the Builder interface such that we can call gRPC's `Register`
method once, on start-up. It uses the same pattern as our resolver
registry where we use the dial target's host (aka "authority"), which is
unique per-agent, to determine which builder to use.
