grpc plugin: Add a plugin providing a gRPC server.

grpc plugin: Add a plugin providing a gRPC server.

gRPC is an open source RPC framework based on protocol buffers. The plugin
currently exposes one RPC end-point for dispatching values to the daemon but
is easy to extend for more functionality.

The plugin is written in C++ and uses an synchronous architecture which should
provide maximum performance. The collectd build system has been extended to
support C++ (11) and protocol buffers (3.0+).