The lookup client with protobuf integration

[adon-at-work]

This is an improvement to https://github.com/yahoo/end-to-end/pull/62

• aligned with the latest lookup verify that check realm in lookup message against the config one
• encapsulated the lookup API as an async client, taking callback and errback
• integrated with decodeIO/protobuf to decode and encode look up response
• integrated with NodeJS during the build process, in order to convert proto file into JSON. that saves the client from converting the proto file every time. protobuf-light can thus be used.
• discovered, and used goog.crypt.base64 to replace with the custom one
• flattenQuorum() now become an one-off operation (instead of running in every lookup), and is initialized when a realm config is loaded
• similarly, public keys of verifiers are instantiated (one-off) as an ed25519 instance when the realm config is first loaded
• simplified / cleaned up the logics in lookupVerify()
• simplified merkle tree by not using present vs. omitted as identifier but directly the type (byte array = omitted vs. node = present)

@andres-erbsen, @daniel-ziegler, @maditya, please help reviewing this. Thanks.

[adon-at-work]

@maditya, let me know if you think what else you think it requires to merge. plan to rebase it just before clicking merge.

The update API is also ready, and can be found at https://github.com/yahoo/end-to-end/tree/feature/update-client-v2. Will create a PR once this current one is merged. Please disable the DKIM check at the server-side. Thanks.

[maditya]

@adon-at-work that's all I have for this PR