Enable allocator to provide proxy with service hash directly

[t-lin]

As was discussed in #43, it was proposed to modify the allocator and proxy to enable the former to provide the hash to the latter through a new environment variable. This will have the following advantages:

1. The proxy won't have to do a hash-lookup on startup, hence potentially saving a bit of time.
2. Solution for the problem described in Service Name Portability service-registry#23

For flexibility, we can still support providing the service name directly, but will need to differentiate them using a flag or something.