airframe-grpc: Support gRPC backend

[xerial]

Added an experimental implementation of gRPC backend:

• Use airframe-codec for gRPC request/response serialization
• Build gRPC server based on Airframe RPC router
• Add gRPC server factory

TODO in another PR:

• Generate gRPC client stub wiht sbt-airframe plugin.
• Support client, server-side, and bidirectional streaming
• Add a gRPC server proxy with airframe-http-finagle for supporting HTTP1

[codecov]

Codecov Report

Merging #1192 into master will increase coverage by 0.08%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #1192      +/-   ##
==========================================
+ Coverage   82.95%   83.03%   +0.08%     
==========================================
  Files         283      288       +5     
  Lines       10962    11018      +56     
  Branches      727      740      +13     
==========================================
+ Hits         9093     9149      +56     
  Misses       1869     1869              

Impacted Files	Coverage Δ
...wvlet/airframe/http/router/HttpRequestMapper.scala	97.56% <ø> (ø)
.../main/scala/wvlet/airframe/http/router/Route.scala	83.33% <ø> (ø)
...la/wvlet/airframe/http/grpc/StringMarshaller.scala	66.66% <66.66%> (ø)
...a/wvlet/airframe/http/grpc/RPCRequestHandler.scala	68.75% <68.75%> (ø)
...in/scala/wvlet/airframe/http/grpc/GrpcServer.scala	76.92% <76.92%> (ø)
...src/main/scala/wvlet/airframe/http/grpc/Grpc.scala	100.00% <100.00%> (ø)
.../wvlet/airframe/http/grpc/GrpcServiceBuilder.scala	100.00% <100.00%> (ø)
... and 5 more

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[takezoe]

@xerial I don't have any comments on the implementation for now. Rather, I would like to ask a question about the motivation of this gRPC backend. In my understanding, one of major advantages of gRPC is interoperability with many other languages based on ProtocolBuffer. Can we offer or leverage this advantage with this backend?

For example, if we can generate Protobuf-IDL from Airframe-RPC endpoint definition, we might be able to benefit from gRPC interoperability by implementing MessagePack marshaller for other languages.

[xerial]

Good point!

The initial motivation is leveraging HTTP2 and server/client-side streaming of gRPC so that we can build fast and interactive RPC services.

Generating a Protobuf IDL from an Airframe RPC interface looks promising and should be easy. What we need to do will be adding a filter for converting Protobuf requests into MessagePack requests (like airframe-codec) to call RPC methods. Then we can leverage existing gRPC client generators for various programming languages. I think most of the people who are already familiar with gRPC will be interested in this direction (Airframe RPC -> Protobuf IDL -> Protobuf gRPC client).

For Scala/Java users, we need to provide sbt-airframe Airframe gRPC client generator, which should be the most efficient client implementation.

[shimamoto]

Looks good so far. In terms of Airframe RPC, I think it's great to use this in almost the same manner.