'nano=true' still works, but any value is now ignored. This is to align
with our lite flag, but also just because It's Cleaner.
Note that this is counter to what javanano does. Javanano requires all
flags have a value and uses true/false for many values.
Client:
* New ManagedChannel abstract class.
* Adding ping to Channel.
* Moving builders and implementations to internal.
Server:
* Added lifecycle management API to Server (mirroring ManagedChannel).
* Moved ServerImpl, AbstractServerBuilder and handler registries to internal.
* New ServerBuilder abstract class (mirroring ManagedChannelBuilder).
Fixes#545